gecko-dev/xpcom/threads/HangMonitor.cpp

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#include "mozilla/HangMonitor.h"
#include "mozilla/Monitor.h"
#include "mozilla/Preferences.h"
#include "nsXULAppAPI.h"
#include "nsThreadUtils.h"

#ifdef MOZ_CRASHREPORTER
#include "nsExceptionHandler.h"
#endif

#ifdef XP_WIN
#include <windows.h>
#endif

namespace mozilla { namespace HangMonitor {

/**
 * A flag which may be set from within a debugger to disable the hang
 * monitor.
 */
volatile bool gDebugDisableHangMonitor = false;

const char kHangMonitorPrefName[] = "hangmonitor.timeout";

// Monitor protects gShutdown and gTimeout, but not gTimestamp which rely on
// being atomically set by the processor; synchronization doesn't really matter
// in this use case.
Monitor* gMonitor;

// The timeout preference, in seconds.
PRInt32 gTimeout;

PRThread* gThread;

// Set when shutdown begins to signal the thread to exit immediately.
bool gShutdown;

// The timestamp of the last event notification, or PR_INTERVAL_NO_WAIT if
// we're currently not processing events.
volatile PRIntervalTime gTimestamp;

// PrefChangedFunc
int
PrefChanged(const char*, void*)
{
  PRInt32 newval = Preferences::GetInt(kHangMonitorPrefName);
  MonitorAutoLock lock(*gMonitor);
  if (newval != gTimeout) {
    gTimeout = newval;
    lock.Notify();
  }

  return 0;
}

void
Crash()
{
  if (gDebugDisableHangMonitor) {
    return;
  }

#ifdef XP_WIN
  if (::IsDebuggerPresent()) {
    return;
  }
#endif

#ifdef MOZ_CRASHREPORTER
  CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("Hang"),
                                     NS_LITERAL_CSTRING("1"));
#endif

  NS_RUNTIMEABORT("HangMonitor triggered");
}

void
ThreadMain(void*)
{
  MonitorAutoLock lock(*gMonitor);

  // In order to avoid issues with the hang monitor incorrectly triggering
  // during a general system stop such as sleeping, the monitor thread must
  // run twice to trigger hang protection.
  PRIntervalTime lastTimestamp = 0;
  int waitCount = 0;

  while (true) {
    if (gShutdown) {
      return; // Exit the thread
    }

    // avoid rereading the volatile value in this loop
    PRIntervalTime timestamp = gTimestamp;

    PRIntervalTime now = PR_IntervalNow();

    if (timestamp != PR_INTERVAL_NO_WAIT &&
        now < timestamp) {
      // 32-bit overflow, reset for another waiting period
      timestamp = 1; // lowest legal PRInterval value
    }

    if (timestamp != PR_INTERVAL_NO_WAIT &&
        timestamp == lastTimestamp &&
        gTimeout > 0) {
      ++waitCount;
      if (waitCount == 2) {
        PRInt32 delay =
          PRInt32(PR_IntervalToSeconds(now - timestamp));
        if (delay > gTimeout) {
          MonitorAutoUnlock unlock(*gMonitor);
          Crash();
        }
      }
    }
    else {
      lastTimestamp = timestamp;
      waitCount = 0;
    }

    PRIntervalTime timeout;
    if (gTimeout <= 0) {
      timeout = PR_INTERVAL_NO_TIMEOUT;
    }
    else {
      timeout = PR_MillisecondsToInterval(gTimeout * 500);
    }
    lock.Wait(timeout);
  }
}

void
Startup()
{
  // The hang detector only runs in chrome processes. If you change this,
  // you must also deal with the threadsafety of AnnotateCrashReport in
  // non-chrome processes!
  if (GeckoProcessType_Default != XRE_GetProcessType())
    return;

  NS_ASSERTION(!gMonitor, "Hang monitor already initialized");
  gMonitor = new Monitor("HangMonitor");

  Preferences::RegisterCallback(PrefChanged, kHangMonitorPrefName, NULL);
  PrefChanged(NULL, NULL);

  // Don't actually start measuring hangs until we hit the main event loop.
  // This potentially misses a small class of really early startup hangs,
  // but avoids dealing with some xpcshell tests and other situations which
  // start XPCOM but don't ever start the event loop.
  Suspend();

  gThread = PR_CreateThread(PR_USER_THREAD,
                            ThreadMain,
                            NULL, PR_PRIORITY_LOW, PR_GLOBAL_THREAD,
                            PR_JOINABLE_THREAD, 0);
}

void
Shutdown()
{
  if (GeckoProcessType_Default != XRE_GetProcessType())
    return;

  NS_ASSERTION(gMonitor, "Hang monitor not started");

  { // Scope the lock we're going to delete later
    MonitorAutoLock lock(*gMonitor);
    gShutdown = true;
    lock.Notify();
  }

  // thread creation could theoretically fail
  if (gThread) {
    PR_JoinThread(gThread);
    gThread = NULL;
  }

  delete gMonitor;
  gMonitor = NULL;
}

void
NotifyActivity()
{
  NS_ASSERTION(NS_IsMainThread(), "HangMonitor::Notify called from off the main thread.");

  // This is not a locked activity because PRTimeStamp is a 32-bit quantity
  // which can be read/written atomically, and we don't want to pay locking
  // penalties here.
  gTimestamp = PR_IntervalNow();
}

void
Suspend()
{
  NS_ASSERTION(NS_IsMainThread(), "HangMonitor::Suspend called from off the main thread.");

  // Because gTimestamp changes this resets the wait count.
  gTimestamp = PR_INTERVAL_NO_WAIT;
}

} } // namespace mozilla::HangMonitor