linux-kernel - Re: [BUG] 4.4.262: infinite loop in futex_unlock

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Date:   Thu, 22 Jul 2021 16:11:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Joe Korty <joe.korty@...current-rt.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Lee Jones <lee.jones@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] 4.4.262: infinite loop in futex_unlock_pi (EAGAIN loop)

On Mon, Jul 19, 2021 at 12:24:18PM -0400, Joe Korty wrote:
> [BUG] 4.4.262: infinite loop in futex_unlock_pi (EAGAIN loop)
> 
>    [ replicator, attached ]
>    [ workaround patch that crudely clears the loop, attached ]
>    [ 4.4.256 does _not_ have this problem, 4.4.262 is known to have it ]
> 
> When a certain, secure-site application is run on 4.4.262, it locks up and
> is unkillable.  Crash(8) and sysrq backtraces show that the application
> is looping in the kernel in futex_unlock_pi.
> 
> Between 4.4.256 and .257, 4.4 got this 4.12 patch backported into it:
> 
>    73d786b ("[PATCH] futex: Rework inconsistent rt_mutex/futex_q state")
> 
> This patch has the following comment:
> 
>    The only problem is that this breaks RT timeliness guarantees. That
>    is, consider the following scenario:
> 
>       T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)
> 
>         CPU0
> 
>         T1
>           lock_pi()
>           queue_me()  <- Waiter is visible
>    
>         preemption
> 
>         T2
>           unlock_pi()
>             loops with -EAGAIN forever
> 
>     Which is undesirable for PI primitives. Future patches will rectify
>     this.
> 
> This describes the situation exactly.  To prove, we developed a little
> kernel patch that, on loop detection, puts a message into the kernel log for
> just the first occurrence, keeps a count of the number of occurrences seen
> since boot, and tries to break out of the loop via usleep_range(1000,1000).
> Note that the patch is not really needed for replication.  It merely shows,
> by 'fixing' the problem, that it really is the EAGAIN loop that triggers
> the lockup.
> 
> Along with this patch, we submit a replicator.  Running this replicator
> with this patch, it can be seen that 4.4.256 does not have the problem.
> 4.4.267 and the latest 4.4, 4.4.275, do.  In addition, 4.9.274 (tested
> w/o the patch) does not have the problem.
> 
> >From this pattern there may be some futex fixup patch that was ported
> back into 4.9 but failed to make it to 4.4.

Odd, I can't seem to find anything that we missed.  Can you dig to see
if there is something that we need to do here so we can resolve this?

thanks,

greg k-h