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cdf71a10c7
Roland Westrelin did a great analysis of a long standing thinko in the return path of futex_lock_pi. While we fixed the lock steal case long ago, which was easy to trigger, we never had a test case which exposed this problem and stupidly never thought about the reverse lock stealing scenario and the return to user space with a stale state. When a blocked tasks returns from rt_mutex_timed_locked without holding the rt_mutex (due to a signal or timeout) and at the same time the task holding the futex is releasing the futex and assigning the ownership of the futex to the returning task, then it might happen that a third task acquires the rt_mutex before the final rt_mutex_trylock() of the returning task happens under the futex hash bucket lock. The returning task returns to user space with ETIMEOUT or EINTR, but the user space futex value is assigned to this task. The task which acquired the rt_mutex fixes the user space futex value right after the hash bucket lock has been released by the returning task, but for a short period of time the user space value is wrong. Detailed description is available at: https://bugzilla.redhat.com/show_bug.cgi?id=400541 The fix for this is the same as we do when the rt_mutex was acquired by a higher priority task via lock stealing from the designated new owner. In that case we already fix the user space value and the internal pi_state up before we return. This mechanism can be used to fixup the above corner case as well. When the returning task, which failed to acquire the rt_mutex, notices that it is the designated owner of the futex, then it fixes up the stale user space value and the pi_state, before returning to user space. This happens with the futex hash bucket lock held, so the task which acquired the rt_mutex is guaranteed to be blocked on the hash bucket lock. We can access the rt_mutex owner, which gives us the pid of the new owner, safely here as the owner is not able to modify (release) it while waiting on the hash bucket lock. Rename the "curr" argument of fixup_pi_state_owner() to "newowner" to avoid confusion with current and add the check for the stale state into the failure path of rt_mutex_trylock() in the return path of unlock_futex_pi(). If the situation is detected use fixup_pi_state_owner() to assign everything to the owner of the rt_mutex. Pointed-out-and-tested-by: Roland Westrelin <roland.westrelin@sun.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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| irq | ||
| power | ||
| time | ||
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| audit.h | ||
| auditfilter.c | ||
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| dma.c | ||
| exec_domain.c | ||
| exit.c | ||
| extable.c | ||
| fork.c | ||
| futex_compat.c | ||
| futex.c | ||
| hrtimer.c | ||
| itimer.c | ||
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| Kconfig.hz | ||
| Kconfig.instrumentation | ||
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| kexec.c | ||
| kfifo.c | ||
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| lockdep_internals.h | ||
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| Makefile | ||
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| module.c | ||
| mutex-debug.c | ||
| mutex-debug.h | ||
| mutex.c | ||
| mutex.h | ||
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| pid.c | ||
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| resource.c | ||
| rtmutex_common.h | ||
| rtmutex-debug.c | ||
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| rtmutex-tester.c | ||
| rtmutex.c | ||
| rtmutex.h | ||
| rwsem.c | ||
| sched_debug.c | ||
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| sched_rt.c | ||
| sched_stats.h | ||
| sched.c | ||
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| signal.c | ||
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| softlockup.c | ||
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| user.c | ||
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