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From: Greg KH <>
To: Samuel Mendoza-Jonas <>
Subject: Re: [PATCH 4.9 4.19] xfs: fix missed wakeup on l_flush_wait
Date: Sun, 2 Aug 2020 08:55:38 +0200
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, Jul 30, 2020 at 02:35:07PM -0700, Samuel Mendoza-Jonas wrote:
> From: Rik van Riel <>
> commit cdea5459ce263fbc963657a7736762ae897a8ae6 upstream
> The code in xlog_wait uses the spinlock to make adding the task to
> the wait queue, and setting the task state to UNINTERRUPTIBLE atomic
> with respect to the waker.
> Doing the wakeup after releasing the spinlock opens up the following
> race condition:
> Task 1					task 2
> add task to wait queue
> 					wake up task
> set task state to UNINTERRUPTIBLE
> This issue was found through code inspection as a result of kworkers
> being observed stuck in UNINTERRUPTIBLE state with an empty
> wait queue. It is rare and largely unreproducable.
> Simply moving the spin_unlock to after the wake_up_all results
> in the waker not being able to see a task on the waitqueue before
> it has set its state to UNINTERRUPTIBLE.
> This bug dates back to the conversion of this code to generic
> waitqueue infrastructure from a counting semaphore back in 2008
> which didn't place the wakeups consistently w.r.t. to the relevant
> spin locks.
> [dchinner: Also fix a similar issue in the shutdown path on
> xc_commit_wait. Update commit log with more details of the issue.]
> Fixes: d748c62367eb ("[XFS] Convert l_flushsema to a sv_t")
> Reported-by: Chris Mason <>
> Signed-off-by: Rik van Riel <>
> Signed-off-by: Dave Chinner <>
> Reviewed-by: Darrick J. Wong <>
> Signed-off-by: Darrick J. Wong <>
> Cc: # 4.9.x-4.19.x
> [modified for contextual change near xlog_state_do_callback()]
> Signed-off-by: Samuel Mendoza-Jonas <>
> Reviewed-by: Frank van der Linden <>
> Reviewed-by: Suraj Jitindar Singh <>
> Reviewed-by: Benjamin Herrenschmidt <>
> Reviewed-by: Anchal Agarwal <>
> ---
> This issue was fixed in v5.4 but didn't appear to make it to stable. The
> fixed commit goes back to v2.6, so backport this to stable kernels
> before v5.4. The only difference is a contextual change at
> 	xlog_state_do_callback(log, XFS_LI_ABORTED, NULL);
> Which in v5.4 is
> 	xlog_state_do_callback(log, true, NULL);

Now queued up, thanks.

greg k-h

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Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30 21:35 Samuel Mendoza-Jonas
2020-08-02  6:55 ` Greg KH [this message]

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