linux-kernel - Re: Question on task_blocks_on_rt

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Message-ID: <20200831232130.GA28456@paulmck-ThinkPad-P72>
Date:   Mon, 31 Aug 2020 16:21:30 -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: Re: Question on task_blocks_on_rt_mutex()

On Mon, Aug 31, 2020 at 03:49:11PM -0700, Paul E. McKenney wrote:
> 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?

One thing I missed is that the low-order bit of ->owner would already
be set by this point.

> 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?

Some breakage elsewhere, presumably...

							Thanx, Paul