Re: [BUG] 4.4.262: infinite loop in futex_unlock_pi (EAGAIN loop)

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jul 27, 2021 at 06:19:50PM -0400, Joe Korty wrote:
> 
>  [ Added missing people to the cc: as listed in MAINTAINERS ]
> 
> On Thu, Jul 22, 2021 at 04:11:41PM +0200, Greg Kroah-Hartman wrote:
> > 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
> 
> 
> Hi Greg,
> 
> 4.12 has these apparently-original patches:
> 
>   73d786b  futex: Rework inconsistent rt_mutex/futex_q state
>   cfafcd1  futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()
> 
> I have verified that the first commit, 73d786b, introduces
> the futex_unlock_pi infinite loop bug into 4.12.  I have
> also verified that the last commit, cfafcd1, fixes the bug.
> 
> 4.9 has had both futex patches backported into it.
> Verified that 4.9.276 does not suffer from the bug.
> 
> 4.4 has had the first patch backported, as 394fc49, but
> not the last.  I have verified that building a kernel at
> 394fc49 and at v4.4.276, the bug is seen, and at 394fc49^,
> the bug is not present.
> 
> The missing commit, cfafcd1 in 4.12, is present in 4.9
> as 13c98b0.  A visual spot-check of 13c98b0, as a patch,
> with kernel/futex.c in 4.4.276 did not find any hunks of
> 13c98b0 present in 4.4.276's kernel/futex.c.

Ok, so what do you recommend be done to resolve this?

thanks,

greg k-h