Re: [BUG -rt] Priority inversion deadlock caused by condvars

From: john stultz
Date: Fri Sep 12 2008 - 18:05:12 EST

Oops, originally sent to the wrong Ulrich.


On Fri, 2008-09-12 at 15:01 -0700, john stultz wrote:
> So we've been seeing application hangs with a very threaded (~8k
> threads) realtime java test. After a fair amount of debugging we found
> most of the SCHED_FIFO threads are blocked in futex_wait(). This raised
> some alarm, since futex_wait isn't priority-inheritance aware.
> After seeing what was going on, Dino came up with a possible deadlock
> case in the pthread_cond_wait() code.
> The problem, as I understand it, assuming there is only one cpu, is if a
> low priority thread is going to call pthread_cond_wait(), it takes the
> associated PI mutex, and calls the function. The glibc implementation
> acquires the condvar's internal non-PI lock, releases the PI mutex and
> tries to block on futex_wait().
> However if a medium priority cpu hog, and a high priority start up while
> the low priority thread holds the mutex, the low priority thread will be
> boosted until it releases that mutex, but not long enough for it to
> release the condvar's internal lock (since the internal lock is not
> priority inherited).
> Then the high priority thread will aquire the mutex, and try to acquire
> the condvar's internal lock (which is still held). However, since we
> also have a medium prio cpu hog, it will block the low priority thread
> from running, and thus block it from releasing the lock.
> And then we're deadlocked.
> Thomas mentioned this is a known problem, but I wanted to send this
> example out so maybe others might become aware.
> The attached test illustrates this hang as described above when bound to
> a single cpu. I believe its correct, but these sorts of tests often have
> their own bugs that create false positives, so please forgive me and let
> me know if you see any problems. :)
> Many thanks to Dino, Ankita and Sripathi for helping to sort out this
> issue.
> To run:
> ./pthread_cond_hang => will PASS (on SMP)
> taskset -c 0 ./pthread_cond_hang => will HANG
> thanks
> -john

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