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Date: Mon, 31 Aug 2020 15:49:11 -0700 From: "Paul E. McKenney" <paulmck@...nel.org> To: peterz@...radead.org, mingo@...hat.com, will@...nel.org Cc: linux-kernel@...r.kernel.org Subject: Question on task_blocks_on_rt_mutex() Hello! The task_blocks_on_rt_mutex() function uses rt_mutex_owner() to take a snapshot of the lock owner right up front. At this point, the ->wait_lock is held, which at first glance prevents the owner from leaving. Except that if there are not yet any waiters (that is, the low-order bit of ->owner is zero), rt_mutex_fastunlock() might locklessly clear the ->owner field. And in that case, it looks like task_blocks_on_rt_mutex() will blithely continue using the ex-owner's task_struct structure, without anything that I can see that prevents the ex-owner from exiting. What am I missing here? The reason that I am looking into this is that locktorture scenario LOCK05 hangs, and does so leaving the torture_rtmutex.waiters field equal to 0x1. This is of course a legal transitional state, but I would not expect it to persist for more than three minutes. Yet it often does. This leads me to believe that there is a way for an unlock to fail to wake up a task concurrently acquiring the lock. This seems to be repaired by later lock acquisitions, and in fact setting the locktorture.stutter module parameter to zero avoids the hang. Except that I first found the above apparently unprotected access to what was recently the owner task. Thoughts? Thanx, Paul
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