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Re: [PATCH 1/2] bdi: mark the bdi flusher busy when being forked

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/2] bdi: mark the bdi flusher busy when being forked
From: Alex Elder <aelder@xxxxxxx>
Date: Wed, 13 Apr 2011 14:29:23 -0500
Cc: xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <1302157196-1988-2-git-send-email-david@xxxxxxxxxxxxx>
References: <1302157196-1988-1-git-send-email-david@xxxxxxxxxxxxx> <1302157196-1988-2-git-send-email-david@xxxxxxxxxxxxx>
Reply-to: aelder@xxxxxxx
On Thu, 2011-04-07 at 16:19 +1000, Dave Chinner wrote: 
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Recetn attempts to use writeback_inode_sb_nr_if_idle() in XFs from
> memory reclaim context have caused deadlocks because memory reclaim
> call be called from a failed allocation during forking a flusher
> thread. The shrinker then attempts to trigger writeback and the bdi
> is considered idle because writeback is not in progress yet and then
> deadlocks because bdi_queue_work() blocks waiting for the
> BDI_Pending bit to clear which will never happen because it needs
> the fork to complete.
> 
> To avoid this deadlock, consider writeback to be in progress if the
> flusher thread is being created. This prevents reclaim from blocking
> waiting for it be forked and hence avoids the deadlock.

I don't believe it matters, but BDI_pending is also set
while a writeback flusher thread is being shut down.

In any case, a handy use of that flag bit.

Reviewed-by: Alex Elder <aelder@xxxxxxx>

> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>


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