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499 lines
16 KiB
C++
499 lines
16 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_DeadlockDetector_h
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#define mozilla_DeadlockDetector_h
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#include "mozilla/Attributes.h"
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#include <stdlib.h>
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#include "prlock.h"
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#include "nsClassHashtable.h"
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#include "nsTArray.h"
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#ifdef NS_TRACE_MALLOC
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# include "nsTraceMalloc.h"
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#endif // ifdef NS_TRACE_MALLOC
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namespace mozilla {
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// FIXME bug 456272: split this off into a convenience API on top of
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// nsStackWalk?
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class NS_COM_GLUE CallStack
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{
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private:
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#ifdef NS_TRACE_MALLOC
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typedef nsTMStackTraceID callstack_id;
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// needs to be a macro to avoid disturbing the backtrace
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# define NS_GET_BACKTRACE() NS_TraceMallocGetStackTrace()
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# define NS_DEADLOCK_DETECTOR_CONSTEXPR
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#else
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typedef void* callstack_id;
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# define NS_GET_BACKTRACE() 0
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# define NS_DEADLOCK_DETECTOR_CONSTEXPR MOZ_CONSTEXPR
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#endif // ifdef NS_TRACE_MALLOC
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callstack_id mCallStack;
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public:
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/**
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* CallStack
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* *ALWAYS* *ALWAYS* *ALWAYS* call this with no arguments. This
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* constructor takes an argument *ONLY* so that |GET_BACKTRACE()|
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* can be evaluated in the stack frame of the caller, rather than
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* that of the constructor.
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*
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* *BEWARE*: this means that calling this constructor with no
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* arguments is not the same as a "default, do-nothing"
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* constructor: it *will* construct a backtrace. This can cause
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* unexpected performance issues.
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*/
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NS_DEADLOCK_DETECTOR_CONSTEXPR
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explicit CallStack(const callstack_id aCallStack = NS_GET_BACKTRACE())
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: mCallStack(aCallStack)
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{
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}
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NS_DEADLOCK_DETECTOR_CONSTEXPR
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CallStack(const CallStack& aFrom)
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: mCallStack(aFrom.mCallStack)
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{
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}
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CallStack& operator=(const CallStack& aFrom)
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{
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mCallStack = aFrom.mCallStack;
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return *this;
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}
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bool operator==(const CallStack& aOther) const
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{
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return mCallStack == aOther.mCallStack;
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}
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bool operator!=(const CallStack& aOther) const
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{
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return mCallStack != aOther.mCallStack;
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}
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/** The "null" callstack. */
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static const CallStack kNone;
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};
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/**
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* DeadlockDetector
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*
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* The following is an approximate description of how the deadlock detector
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* works.
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*
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* The deadlock detector ensures that all blocking resources are
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* acquired according to a partial order P. One type of blocking
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* resource is a lock. If a lock l1 is acquired (locked) before l2,
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* then we say that |l1 <_P l2|. The detector flags an error if two
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* locks l1 and l2 have an inconsistent ordering in P; that is, if
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* both |l1 <_P l2| and |l2 <_P l1|. This is a potential error
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* because a thread acquiring l1,l2 according to the first order might
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* race with a thread acquiring them according to the second order.
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* If this happens under the right conditions, then the acquisitions
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* will deadlock.
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*
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* This deadlock detector doesn't know at compile-time what P is. So,
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* it tries to discover the order at run time. More precisely, it
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* finds <i>some</i> order P, then tries to find chains of resource
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* acquisitions that violate P. An example acquisition sequence, and
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* the orders they impose, is
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* l1.lock() // current chain: [ l1 ]
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* // order: { }
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*
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* l2.lock() // current chain: [ l1, l2 ]
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* // order: { l1 <_P l2 }
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*
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* l3.lock() // current chain: [ l1, l2, l3 ]
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* // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3 }
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* // (note: <_P is transitive, so also |l1 <_P l3|)
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*
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* l2.unlock() // current chain: [ l1, l3 ]
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* // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3 }
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* // (note: it's OK, but weird, that l2 was unlocked out
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* // of order. we still have l1 <_P l3).
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*
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* l2.lock() // current chain: [ l1, l3, l2 ]
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* // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3,
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* l3 <_P l2 (!!!) }
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* BEEP BEEP! Here the detector will flag a potential error, since
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* l2 and l3 were used inconsistently (and potentially in ways that
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* would deadlock).
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*/
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template<typename T>
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class DeadlockDetector
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{
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public:
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/**
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* ResourceAcquisition
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* Consists simply of a resource and the calling context from
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* which it was acquired. We pack this information together so
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* that it can be returned back to the caller when a potential
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* deadlock has been found.
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*/
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struct ResourceAcquisition
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{
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const T* mResource;
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CallStack mCallContext;
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explicit ResourceAcquisition(const T* aResource,
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const CallStack aCallContext = CallStack::kNone)
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: mResource(aResource)
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, mCallContext(aCallContext)
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{
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}
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ResourceAcquisition(const ResourceAcquisition& aFrom)
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: mResource(aFrom.mResource)
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, mCallContext(aFrom.mCallContext)
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{
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}
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ResourceAcquisition& operator=(const ResourceAcquisition& aFrom)
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{
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mResource = aFrom.mResource;
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mCallContext = aFrom.mCallContext;
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return *this;
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}
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};
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typedef nsTArray<ResourceAcquisition> ResourceAcquisitionArray;
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private:
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struct OrderingEntry;
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typedef nsTArray<OrderingEntry*> HashEntryArray;
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typedef typename HashEntryArray::index_type index_type;
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typedef typename HashEntryArray::size_type size_type;
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static const index_type NoIndex = HashEntryArray::NoIndex;
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/**
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* Value type for the ordering table. Contains the other
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* resources on which an ordering constraint |key < other|
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* exists. The catch is that we also store the calling context at
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* which the other resource was acquired; this improves the
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* quality of error messages when potential deadlock is detected.
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*/
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struct OrderingEntry
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{
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OrderingEntry(const T* aResource)
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: mFirstSeen(CallStack::kNone)
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, mOrderedLT() // FIXME bug 456272: set to empirical dep size?
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, mExternalRefs()
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, mResource(aResource)
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{
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}
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~OrderingEntry()
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{
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}
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size_t
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SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
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{
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size_t n = aMallocSizeOf(this);
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n += mOrderedLT.SizeOfExcludingThis(aMallocSizeOf);
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n += mExternalRefs.SizeOfExcludingThis(aMallocSizeOf);
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n += mResource->SizeOfIncludingThis(aMallocSizeOf);
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return n;
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}
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CallStack mFirstSeen; // first site from which the resource appeared
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HashEntryArray mOrderedLT; // this <_o Other
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HashEntryArray mExternalRefs; // hash entries that reference this
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const T* mResource;
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};
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// Throwaway RAII lock to make the following code safer.
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struct PRAutoLock
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{
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explicit PRAutoLock(PRLock* aLock) : mLock(aLock) { PR_Lock(mLock); }
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~PRAutoLock() { PR_Unlock(mLock); }
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PRLock* mLock;
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};
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public:
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static const uint32_t kDefaultNumBuckets;
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/**
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* DeadlockDetector
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* Create a new deadlock detector.
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*
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* @param aNumResourcesGuess Guess at approximate number of resources
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* that will be checked.
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*/
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explicit DeadlockDetector(uint32_t aNumResourcesGuess = kDefaultNumBuckets)
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: mOrdering(aNumResourcesGuess)
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{
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mLock = PR_NewLock();
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if (!mLock) {
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NS_RUNTIMEABORT("couldn't allocate deadlock detector lock");
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}
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}
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/**
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* ~DeadlockDetector
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*
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* *NOT* thread safe.
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*/
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~DeadlockDetector()
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{
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PR_DestroyLock(mLock);
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}
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static size_t
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SizeOfEntryExcludingThis(const T* aKey, const nsAutoPtr<OrderingEntry>& aEntry,
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MallocSizeOf aMallocSizeOf, void* aUserArg)
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{
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// NB: Key is accounted for in the entry.
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size_t n = aEntry->SizeOfIncludingThis(aMallocSizeOf);
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return n;
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}
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size_t
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SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
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{
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size_t n = aMallocSizeOf(this);
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{
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PRAutoLock _(mLock);
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n += mOrdering.SizeOfExcludingThis(SizeOfEntryExcludingThis, aMallocSizeOf);
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}
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return n;
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}
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/**
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* Add
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* Make the deadlock detector aware of |aResource|.
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*
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* WARNING: The deadlock detector owns |aResource|.
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*
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* Thread safe.
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*
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* @param aResource Resource to make deadlock detector aware of.
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*/
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void Add(T* aResource)
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{
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PRAutoLock _(mLock);
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mOrdering.Put(aResource, new OrderingEntry(aResource));
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}
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void Remove(T* aResource)
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{
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PRAutoLock _(mLock);
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OrderingEntry* entry = mOrdering.Get(aResource);
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// Iterate the external refs and remove the entry from them.
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HashEntryArray& refs = entry->mExternalRefs;
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for (index_type i = 0; i < refs.Length(); i++) {
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refs[i]->mOrderedLT.RemoveElementSorted(entry);
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}
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// Iterate orders and remove this entry from their refs.
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HashEntryArray& orders = entry->mOrderedLT;
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for (index_type i = 0; i < orders.Length(); i++) {
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orders[i]->mExternalRefs.RemoveElementSorted(entry);
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}
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// Now the entry can be safely removed.
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mOrdering.Remove(aResource);
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delete aResource;
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}
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/**
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* CheckAcquisition This method is called after acquiring |aLast|,
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* but before trying to acquire |aProposed| from |aCallContext|.
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* It determines whether actually trying to acquire |aProposed|
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* will create problems. It is OK if |aLast| is nullptr; this is
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* interpreted as |aProposed| being the thread's first acquisition
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* of its current chain.
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*
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* Iff acquiring |aProposed| may lead to deadlock for some thread
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* interleaving (including the current one!), the cyclical
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* dependency from which this was deduced is returned. Otherwise,
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* 0 is returned.
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*
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* If a potential deadlock is detected and a resource cycle is
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* returned, it is the *caller's* responsibility to free it.
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*
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* Thread safe.
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*
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* @param aLast Last resource acquired by calling thread (or 0).
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* @param aProposed Resource calling thread proposes to acquire.
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* @param aCallContext Calling context whence acquisiton request came.
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*/
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ResourceAcquisitionArray* CheckAcquisition(const T* aLast,
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const T* aProposed,
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const CallStack& aCallContext)
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{
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NS_ASSERTION(aProposed, "null resource");
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PRAutoLock _(mLock);
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OrderingEntry* proposed = mOrdering.Get(aProposed);
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NS_ASSERTION(proposed, "missing ordering entry");
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if (CallStack::kNone == proposed->mFirstSeen) {
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proposed->mFirstSeen = aCallContext;
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}
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if (!aLast) {
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// don't check if |0 < aProposed|; just vamoose
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return 0;
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}
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OrderingEntry* current = mOrdering.Get(aLast);
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NS_ASSERTION(current, "missing ordering entry");
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// this is the crux of the deadlock detector algorithm
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if (current == proposed) {
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// reflexive deadlock. fastpath b/c InTransitiveClosure is
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// not applicable here.
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ResourceAcquisitionArray* cycle = new ResourceAcquisitionArray();
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if (!cycle) {
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NS_RUNTIMEABORT("can't allocate dep. cycle array");
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}
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cycle->AppendElement(ResourceAcquisition(current->mResource,
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current->mFirstSeen));
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cycle->AppendElement(ResourceAcquisition(aProposed,
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aCallContext));
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return cycle;
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}
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if (InTransitiveClosure(current, proposed)) {
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// we've already established |aLast < aProposed|. all is well.
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return 0;
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}
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if (InTransitiveClosure(proposed, current)) {
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// the order |aProposed < aLast| has been deduced, perhaps
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// transitively. we're attempting to violate that
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// constraint by acquiring resources in the order
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// |aLast < aProposed|, and thus we may deadlock under the
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// right conditions.
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ResourceAcquisitionArray* cycle = GetDeductionChain(proposed, current);
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// show how acquiring |aProposed| would complete the cycle
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cycle->AppendElement(ResourceAcquisition(aProposed,
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aCallContext));
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return cycle;
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}
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// |aLast|, |aProposed| are unordered according to our
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// poset. this is fine, but we now need to add this
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// ordering constraint.
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current->mOrderedLT.InsertElementSorted(proposed);
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proposed->mExternalRefs.InsertElementSorted(current);
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return 0;
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}
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/**
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* Return true iff |aTarget| is in the transitive closure of |aStart|
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* over the ordering relation `<_this'.
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*
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* @precondition |aStart != aTarget|
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*/
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bool InTransitiveClosure(const OrderingEntry* aStart,
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const OrderingEntry* aTarget) const
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{
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// NB: Using a static comparator rather than default constructing one shows
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// a 9% improvement in scalability tests on some systems.
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static nsDefaultComparator<const OrderingEntry*, const OrderingEntry*> comp;
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if (aStart->mOrderedLT.BinaryIndexOf(aTarget, comp) != NoIndex) {
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return true;
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}
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index_type i = 0;
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size_type len = aStart->mOrderedLT.Length();
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for (const OrderingEntry* const* it = aStart->mOrderedLT.Elements(); i < len; ++i, ++it) {
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if (InTransitiveClosure(*it, aTarget)) {
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return true;
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}
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}
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return false;
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}
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/**
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* Return an array of all resource acquisitions
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* aStart <_this r1 <_this r2 <_ ... <_ aTarget
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* from which |aStart <_this aTarget| was deduced, including
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* |aStart| and |aTarget|.
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*
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* Nb: there may be multiple deductions of |aStart <_this
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* aTarget|. This function returns the first ordering found by
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* depth-first search.
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*
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* Nb: |InTransitiveClosure| could be replaced by this function.
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* However, this one is more expensive because we record the DFS
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* search stack on the heap whereas the other doesn't.
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*
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* @precondition |aStart != aTarget|
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*/
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ResourceAcquisitionArray* GetDeductionChain(const OrderingEntry* aStart,
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const OrderingEntry* aTarget)
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{
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ResourceAcquisitionArray* chain = new ResourceAcquisitionArray();
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if (!chain) {
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NS_RUNTIMEABORT("can't allocate dep. cycle array");
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}
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chain->AppendElement(ResourceAcquisition(aStart->mResource,
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aStart->mFirstSeen));
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NS_ASSERTION(GetDeductionChain_Helper(aStart, aTarget, chain),
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"GetDeductionChain called when there's no deadlock");
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return chain;
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}
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// precondition: |aStart != aTarget|
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// invariant: |aStart| is the last element in |aChain|
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bool GetDeductionChain_Helper(const OrderingEntry* aStart,
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const OrderingEntry* aTarget,
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ResourceAcquisitionArray* aChain)
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{
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if (aStart->mOrderedLT.BinaryIndexOf(aTarget) != NoIndex) {
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aChain->AppendElement(ResourceAcquisition(aTarget->mResource,
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aTarget->mFirstSeen));
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return true;
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}
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index_type i = 0;
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size_type len = aStart->mOrderedLT.Length();
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for (const OrderingEntry* const* it = aStart->mOrderedLT.Elements(); i < len; ++i, ++it) {
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aChain->AppendElement(ResourceAcquisition((*it)->mResource,
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(*it)->mFirstSeen));
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if (GetDeductionChain_Helper(*it, aTarget, aChain)) {
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return true;
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}
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aChain->RemoveElementAt(aChain->Length() - 1);
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}
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return false;
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}
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/**
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* The partial order on resource acquisitions used by the deadlock
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* detector.
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*/
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nsClassHashtable<nsPtrHashKey<const T>, OrderingEntry> mOrdering;
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/**
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* Protects contentious methods.
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* Nb: can't use mozilla::Mutex since we are used as its deadlock
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* detector.
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*/
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PRLock* mLock;
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private:
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DeadlockDetector(const DeadlockDetector& aDD) MOZ_DELETE;
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DeadlockDetector& operator=(const DeadlockDetector& aDD) MOZ_DELETE;
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};
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template<typename T>
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// FIXME bug 456272: tune based on average workload
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const uint32_t DeadlockDetector<T>::kDefaultNumBuckets = 32;
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} // namespace mozilla
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#endif // ifndef mozilla_DeadlockDetector_h
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