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Re: [PATCH 1/8] xfs: check for buffer errors before waiting

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH 1/8] xfs: check for buffer errors before waiting
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 30 Mar 2012 08:10:25 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4F74B229.6030707@xxxxxxx>
References: <1333023835-12856-1-git-send-email-david@xxxxxxxxxxxxx> <1333023835-12856-2-git-send-email-david@xxxxxxxxxxxxx> <4F74B229.6030707@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Mar 29, 2012 at 02:04:09PM -0500, Mark Tinguely wrote:
> On 03/29/12 07:23, Dave Chinner wrote:
> >From: Dave Chinner<dchinner@xxxxxxxxxx>
> >
> >If we call xfs_buf_iowait() on a buffer that failed dispatch due to
> >an IO error, it will wait forever for an Io that does not exist.
> >This is hndled in xfs_buf_read, but there is other code that calls
> >xfs_buf_iowait directly that doesn't.
> >
> >Rather than make the call sites have to handle checking for dispatch
> >errors and then checking for completion errors, make
> >xfs_buf_iowait() check for dispatch errors on the buffer before
> >waiting. This means we handle both dispatch and completion errors
> >with one set of error handling at the caller sites.
> >
> >Signed-off-by: Dave Chinner<dchinner@xxxxxxxxxx>
> >---
> 
> >diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
> >index 396e3bf..64ed6ff 100644
> >--- a/fs/xfs/xfs_log_recover.c
> >+++ b/fs/xfs/xfs_log_recover.c
> >@@ -179,6 +179,7 @@ xlog_bread_noalign(
> >     XFS_BUF_SET_ADDR(bp, log->l_logBBstart + blk_no);
> >     XFS_BUF_READ(bp);
> >     XFS_BUF_SET_COUNT(bp, BBTOB(nbblks));
> >+    bp->b_error = 0;
> >
> >     xfsbdstrat(log->l_mp, bp);
> >     error = xfs_buf_iowait(bp);
> >@@ -266,6 +267,7 @@ xlog_bwrite(
> >     xfs_buf_hold(bp);
> >     xfs_buf_lock(bp);
> >     XFS_BUF_SET_COUNT(bp, BBTOB(nbblks));
> >+    bp->b_error = 0;
> >
> >     error = xfs_bwrite(bp);
> >     if (error)
> 
> Just curious, were these needed for a particular reason?

If the previous user of the buffer got an error, it is not
guaranteed to be cleared because the buffer is not re-initialised.
i.e. it's an uncached buffer that we control completely and reuse
from IO to IO with just a reset of the bno and length. If b_error is
non zero here, then the IO can fail because nothing else clears the
error in the dispatch path....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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