From: Greg Kroah-Hartman <email@example.com> To: Joe Korty <firstname.lastname@example.org> Cc: Peter Zijlstra <email@example.com>, Lee Jones <firstname.lastname@example.org>, Steven Rostedt <email@example.com>, Thomas Gleixner <firstname.lastname@example.org>, Sebastian Andrzej Siewior <email@example.com>, LKML <firstname.lastname@example.org> Subject: Re: [BUG] 4.4.262: infinite loop in futex_unlock_pi (EAGAIN loop) Date: Thu, 22 Jul 2021 16:11:41 +0200 [thread overview] Message-ID: <YPl8nfZBjgmSnE7N@kroah.com> (raw) In-Reply-To: <20210719162418.GA28003@zipoli.concurrent-rt.com> 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
next prev parent reply other threads:[~2021-07-22 14:11 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-19 16:24 Joe Korty 2021-07-22 14:11 ` Greg Kroah-Hartman [this message] 2021-07-27 22:19 ` Joe Korty 2021-07-28 6:07 ` Greg Kroah-Hartman 2021-07-28 13:51 ` Joe Korty 2021-07-28 16:15 ` Greg Kroah-Hartman
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