From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay1.corp.sgi.com [137.38.102.111]) by oss.sgi.com (Postfix) with ESMTP id 6AF977F37 for ; Mon, 14 Sep 2015 08:25:05 -0500 (CDT) Received: from cuda.sgi.com (cuda1.sgi.com [192.48.157.11]) by relay1.corp.sgi.com (Postfix) with ESMTP id 3D4D08F8037 for ; Mon, 14 Sep 2015 06:25:02 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by cuda.sgi.com with ESMTP id CFEZNsaPzcIgGxqI (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO) for ; Mon, 14 Sep 2015 06:24:57 -0700 (PDT) Date: Mon, 14 Sep 2015 09:24:55 -0400 From: Brian Foster Subject: Re: [PATCH] xfs: add missing ilock around dio write last extent alignment Message-ID: <20150914132455.GA22770@bfoster.bfoster> References: <1441809812-60175-1-git-send-email-bfoster@redhat.com> <20150913235835.GV26895@dastard> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20150913235835.GV26895@dastard> List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com To: Dave Chinner Cc: David Jeffery , xfs@oss.sgi.com On Mon, Sep 14, 2015 at 09:58:35AM +1000, Dave Chinner wrote: > On Wed, Sep 09, 2015 at 10:43:32AM -0400, Brian Foster wrote: > > The iomap codepath (via get_blocks()) acquires and release the inode > > lock in the case of a direct write that requires block allocation. This > > is because xfs_iomap_write_direct() allocates a transaction, which means > > the ilock must be dropped and reacquired after the transaction is > > allocated and reserved. > > > > xfs_iomap_write_direct() invokes xfs_iomap_eof_align_last_fsb() before > > the transaction is created and thus before the ilock is reacquired. This > > can lead to calls to xfs_iread_extents() and reads of the in-core extent > > list without any synchronization (via xfs_bmap_eof() and > > xfs_bmap_last_extent()). xfs_iread_extents() assert fails if the ilock > > is not held, but this is not currently seen in practice as the current > > callers had already invoked xfs_bmapi_read(). > > > > What has been seen in practice are reports of crashes down in the > > xfs_bmap_eof() codepath on direct writes due to seemingly bogus pointer > > references from xfs_iext_get_ext(). While an explicit reproducer is not > > currently available to confirm the cause of the problem, crash analysis > > and code inspection from David Jeffrey had identified the insufficient > > locking. > > > > xfs_iomap_eof_align_last_fsb() is called from other contexts with the > > inode lock already held. __xfs_get_blocks() acquires and drops the ilock > > with variable flags. Therefore, take the simple approach to cycle ilock > > around the last extent alignment call from xfs_iomap_write_direct(). > > > > Reported-by: David Jeffery > > Signed-off-by: Brian Foster > > --- > > fs/xfs/xfs_iomap.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c > > index 1f86033..4d7534e 100644 > > --- a/fs/xfs/xfs_iomap.c > > +++ b/fs/xfs/xfs_iomap.c > > @@ -142,7 +142,9 @@ xfs_iomap_write_direct( > > offset_fsb = XFS_B_TO_FSBT(mp, offset); > > last_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)(offset + count))); > > if ((offset + count) > XFS_ISIZE(ip)) { > > + xfs_ilock(ip, XFS_ILOCK_EXCL); > > error = xfs_iomap_eof_align_last_fsb(mp, ip, extsz, &last_fsb); > > + xfs_iunlock(ip, XFS_ILOCK_EXCL); > > XFS_ILOCK_SHARED? > I suspect that is technically sufficient in this particular call path given that we've called xfs_bmapi_read(). The problem is that there is a call to xfs_iread_extents() buried a few calls deep in xfs_bmap_last_extent(). My understanding is that we need the exclusive lock because it's not safe for multiple threads to populate the in-core extent list at the same time, so I don't really want to replace the existing race with a landmine should the context happen to change in the future. > Also, looking at __xfs_get_blocks(), we drop the ilock immediately > before calling xfs_iomap_write_direct(), which we already hold in > shared mode for the xfs_bmapi_read() for direct IO. > > Can we push that lock dropping into xfs_iomap_write_direct() after > we've done the xfs_iomap_eof_align_last_fsb() call and before we do > transaction reservations so we don't need an extra lock round-trip > here? e.g. xfs_iomap_write_delay() is called under the lock context > held by __xfs_get_blocks().... > That was my initial thought when looking at this code... e.g., to just carry the lock over and drop it prior to transaction setup. I didn't go that route because __xfs_get_blocks() uses a variable locking mode and it seemed ugly to pass along the lock mode to xfs_iomap_direct_write(). Further, given the above it also looked like we'd have to check and cycle the ilock EXCL if it were ILOCK_SHARED. Finally, xfs_iomap_direct_write() has a call to xfs_qm_dqattach() which itself acquires ILOCK_EXCL. Looking at xfs_iomap_write_delay(), we do have a dqattach_locked() variant but it also expects to have ILOCK_EXCL. Hmm, so in the common case both the extent list and a quota are handled once and thus the only notable lock cycle is the align_last_fsb() case. I think we could do something like this: - Create a shared lock safe variant of xfs_iomap_eof_align_last_fsb() to be called from xfs_iomap_write_direct(). - __xfs_get_blocks() continues to call xfs_ilock_data_map_shared(), but unconditionally demotes XFS_ILOCK_EXCL to XFS_ILOCK_SHARED before calling xfs_iomap_write_direct(). - xfs_iomap_write_direct() moves the xfs_qm_dqattach() call to immediately before the transaction allocation. E.g., it executes the existing align_last_fsb() bits and whatnot under XFS_ILOCK_SHARED, drops the lock, potentially attaches the quota and carries on as normal with the transaction. The only thing I'm not sure about is the shared lock safe version of xfs_iomap_eof_align_last_fsb(). The xfs_iread_extents() call is a few calls deep and xfs_bmap_last_extent() is called from other contexts. I suppose we could call it as is and pull up an assert to check for XFS_IFEXTENTS such that the situation is explicitly documented in the appropriate context (we do already have the assert in xfs_iread_extents() if it were called). Also, I take it we can safely assume the in-core extent list is still around if we still hold the lock from the xfs_bmapi_read() call. Thoughts? I guess I'll float another patch... Brian > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com > > _______________________________________________ > xfs mailing list > xfs@oss.sgi.com > http://oss.sgi.com/mailman/listinfo/xfs _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs