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Re: [PATCH] xfs: fix error handling for synchronous writes

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix error handling for synchronous writes
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 10 Jan 2011 11:21:09 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110107130223.GB12134@xxxxxxxxxxxxx>
References: <20110107130223.GB12134@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri, Jan 07, 2011 at 08:02:23AM -0500, Christoph Hellwig wrote:
> If we get an IO error on a synchronous superblock write, we attach an
> error release function to it so that when the last reference goes away
> the release function is called and the buffer is invalidated and
> unlocked. The buffer is left locked until the release function is
> called so that other concurrent users of the buffer will be locked out
> until the buffer error is fully processed.
> 
> Unfortunately, for the superblock buffer the filesyetm itself holds a
> reference to the buffer which prevents the reference count from
> dropping to zero and the release function being called. As a result,
> once an IO error occurs on a sync write, the buffer will never be
> unlocked and all future attempts to lock the buffer will hang.
> 
> To make matters worse, this problems is not unique to such buffers;
> if there is a concurrent _xfs_buf_find() running, the lookup will grab
> a reference to the buffer and then wait on the buffer lock, preventing
> the reference count from ever falling to zero and hence unlocking the
> buffer.
> 
> As such, the whole b_relse function implementation is broken because it
> cannot rely on the buffer reference count falling to zero to unlock the
> errored buffer. The synchronous write error path is the only path that
> uses this callback - it is used to ensure that the synchronous waiter
> gets the buffer error before the error state is cleared from the buffer
> by the release function.
> 
> Given that the only sychronous buffer writes now go through xfs_bwrite
> and the error path in question can only occur for a write of a dirty,
> logged buffer, we can move most of the b_relse processing to happen
> inline in xfs_buf_iodone_callbacks, just like a normal I/O completion.
> In addition to that we make sure the error is not cleared in
> xfs_buf_iodone_callbacks, so that xfs_bwrite can reliably check it.
> Given that xfs_bwrite keeps the buffer locked until it has waited for
> it and checked the error this allows to reliably propagate the error
> to the caller, and make sure that the buffer is reliably unlocked.
> 
> Given that xfs_buf_iodone_callbacks was the only instance of the
> b_relse callback we can remove it entirely.
> 
> Based on earlier patches by Dave Chinner and Ajeet Yadav.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Reported-by: Ajeet Yadav <ajeet.yadav.77@xxxxxxxxx>

Passes xfsqa fine here.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

-- 
Dave Chinner
david@xxxxxxxxxxxxx

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