linux-kernel - Re: [PATCH] xfs: Wake CIL push waiters more reliably

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Date:   Mon, 15 Feb 2021 14:36:38 +0100
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Dave Chinner <david@...morbit.com>,
        Brian Foster <bfoster@...hat.com>
Cc:     linux-xfs@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        it+linux-xfs@...gen.mpg.de
Subject: Re: [PATCH] xfs: Wake CIL push waiters more reliably

On 13.01.21 22:53, Dave Chinner wrote:
> [...]
> I agree that a throttling fix is needed, but I'm trying to
> understand the scope and breadth of the problem first instead of
> jumping the gun and making the wrong fix for the wrong reasons that
> just papers over the underlying problems that the throttling bug has
> made us aware of...

Are you still working on this?

If it takes more time to understand the potential underlying problem, the fix for the problem at hand should be applied.

This is a real world problem, accidentally found in the wild. It appears very rarely, but it freezes a filesystem or the whole system. It exists in 5.7 , 5.8 , 5.9 , 5.10 and 5.11 and is caused by c7f87f3984cf ("xfs: fix use-after-free on CIL context on shutdown") which silently added a condition to the wakeup. The condition is based on a wrong assumption.

Why is this "papering over"? If a reminder was needed, there were better ways than randomly hanging the system.

Why is

     if (ctx->space_used >= XLOG_CIL_BLOCKING_SPACE_LIMIT(log))
         wake_up_all(&cil->xc_push_wait);

, which doesn't work reliably, preferable to

     if (waitqueue_active(&cil->xc_push_wait))
         wake_up_all(&cil->xc_push_wait);

which does?

Best
   Donald

> Cheers,
> 
> Dave