From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay3.corp.sgi.com [198.149.34.15]) by oss.sgi.com (Postfix) with ESMTP id 368A97F47 for ; Wed, 16 Sep 2015 17:35:27 -0500 (CDT) Received: from cuda.sgi.com (cuda2.sgi.com [192.48.176.25]) by relay3.corp.sgi.com (Postfix) with ESMTP id B6F64AC004 for ; Wed, 16 Sep 2015 15:35:26 -0700 (PDT) Received: from ipmail06.adl2.internode.on.net (ipmail06.adl2.internode.on.net [150.101.137.129]) by cuda.sgi.com with ESMTP id WN0oJRbbp8DFFCMn for ; Wed, 16 Sep 2015 15:35:24 -0700 (PDT) Date: Thu, 17 Sep 2015 08:34:35 +1000 From: Dave Chinner Subject: Re: [PATCH] xfs: add missing ilock around dio write last extent alignment Message-ID: <20150916223435.GW26895@dastard> References: <1441809812-60175-1-git-send-email-bfoster@redhat.com> <20150913235835.GV26895@dastard> <20150914132455.GA22770@bfoster.bfoster> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20150914132455.GA22770@bfoster.bfoster> 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: Brian Foster Cc: David Jeffery , xfs@oss.sgi.com On Mon, Sep 14, 2015 at 09:24:55AM -0400, Brian Foster wrote: > 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(). Sure. > 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. yes, but that can't happen here because we are guaranteed to have the extent list in memory because we've alreay called xfs_bmapi_read() and that will populate the extent list with the appropriate lock held. > > 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, No, because the __xfs_get_blocks code calls xfs_ilock_data_map_shared() for direct IO, so already holds the correct lock for populating the extent list (not that this matters here). > 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. That can be moved to after we've calculated the last extent. i.e. to just before we start the transaction.... > The only thing I'm not sure about is the shared lock safe version of > xfs_iomap_eof_align_last_fsb(). All the callers are guaranteed to have first populated the extent list, so this should be safe. If you are really worried, add an assert that verifies either ILOCK_EXCL or (ILOCK_SHARED && extents read 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... Once the extents are read in, they are in memory until the inode is reclaimed. That won't happen while we have active references to it. :) Cheers, Dave. -- Dave Chinner david@fromorbit.com _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs