Re: [Bug 207053] fsfreeze deadlock on XFS (the FIFREEZE ioctl and subsequent FITHAW hang indefinitely)

[Brian Foster <bfoster@redhat.com>]

On Tue, Apr 07, 2020 at 08:17:38AM -0700, Darrick J. Wong wrote:
> On Tue, Apr 07, 2020 at 09:18:12AM -0400, Brian Foster wrote:
> > On Tue, Apr 07, 2020 at 06:41:31AM +0000, bugzilla-daemon@bugzilla.kernel.org wrote:
> > > https://bugzilla.kernel.org/show_bug.cgi?id=207053
> > > 
> > > --- Comment #2 from Paul Furtado (paulfurtado91@gmail.com) ---
> > > Hi Dave,
> > > 
> > > Just had another case of this crop up and I was able to get the blocked tasks
> > > output before automation killed the server. Because the log was too large to
> > > attach, I've pasted the output into a github gist here:
> > > https://gist.githubusercontent.com/PaulFurtado/c9bade038b8a5c7ddb53a6e10def058f/raw/ee43926c96c0d6a9ec81a648754c1af599ef0bdd/sysrq_w.log
> > > 
> > 
> > Hm, so it looks like this is stuck between freeze:
> > 
> > [377279.630957] fsfreeze        D    0 46819  46337 0x00004084
> > [377279.634910] Call Trace:
> > [377279.637594]  ? __schedule+0x292/0x6f0
> > [377279.640833]  ? xfs_xattr_get+0x51/0x80 [xfs]
> > [377279.644287]  schedule+0x2f/0xa0
> > [377279.647286]  schedule_timeout+0x1dd/0x300
> > [377279.650661]  wait_for_completion+0x126/0x190
> > [377279.654154]  ? wake_up_q+0x80/0x80
> > [377279.657277]  ? work_busy+0x80/0x80
> > [377279.660375]  __flush_work+0x177/0x1b0
> > [377279.663604]  ? worker_attach_to_pool+0x90/0x90
> > [377279.667121]  __cancel_work_timer+0x12b/0x1b0
> > [377279.670571]  ? rcu_sync_enter+0x8b/0xd0
> > [377279.673864]  xfs_stop_block_reaping+0x15/0x30 [xfs]
> > [377279.677585]  xfs_fs_freeze+0x15/0x40 [xfs]
> > [377279.680950]  freeze_super+0xc8/0x190
> > [377279.684086]  do_vfs_ioctl+0x510/0x630
> > ...
> > 
> > ... and the eofblocks scanner:
> > 
> > [377279.422496] Workqueue: xfs-eofblocks/nvme13n1 xfs_eofblocks_worker [xfs]
> > [377279.426971] Call Trace:
> > [377279.429662]  ? __schedule+0x292/0x6f0
> > [377279.432839]  schedule+0x2f/0xa0
> > [377279.435794]  rwsem_down_read_slowpath+0x196/0x530
> > [377279.439435]  ? kmem_cache_alloc+0x152/0x1f0
> > [377279.442834]  ? __percpu_down_read+0x49/0x60
> > [377279.446242]  __percpu_down_read+0x49/0x60
> > [377279.449586]  __sb_start_write+0x5b/0x60
> > [377279.452869]  xfs_trans_alloc+0x152/0x160 [xfs]
> > [377279.456372]  xfs_free_eofblocks+0x12d/0x1f0 [xfs]
> > [377279.460014]  xfs_inode_free_eofblocks+0x128/0x1a0 [xfs]
> > [377279.463903]  ? xfs_inode_ag_walk_grab+0x5f/0x90 [xfs]
> > [377279.467680]  xfs_inode_ag_walk.isra.17+0x1a7/0x410 [xfs]
> > [377279.471567]  ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs]
> > [377279.475620]  ? kvm_sched_clock_read+0xd/0x20
> > [377279.479059]  ? sched_clock+0x5/0x10
> > [377279.482184]  ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs]
> > [377279.486234]  ? radix_tree_gang_lookup_tag+0xa8/0x100
> > [377279.489974]  ? __xfs_inode_clear_blocks_tag+0x120/0x120 [xfs]
> > [377279.494041]  xfs_inode_ag_iterator_tag+0x73/0xb0 [xfs]
> > [377279.497859]  xfs_eofblocks_worker+0x29/0x40 [xfs]
> > [377279.501484]  process_one_work+0x195/0x380
> > ...
> > 
> > The immediate issue is likely that the eofblocks transaction is not
> > NOWRITECOUNT (same for the cowblocks scanner, btw), but the problem with
> > doing that is these helpers are called from other contexts outside of
> > the background scanners.
> > 
> > Perhaps what we need to do here is let these background scanners acquire
> > a superblock write reference, similar to what Darrick recently added to
> > scrub..? We'd have to do that from the scanner workqueue task, so it
> > would probably need to be a trylock so we don't end up in a similar
> > situation as above. I.e., we'd either get the reference and cause freeze
> > to wait until it's dropped or bail out if freeze has already stopped the
> > transaction subsystem. Thoughts?
> 
> Hmm, I had a whole gigantic series to refactor all the speculative
> preallocation gc work into a single thread + radix tree tag; I'll see if
> that series actually fixed this problem too.
> 
> But yes, all background threads that run transactions need to have
> freezer protection.
> 

So something like the following in the meantime, assuming we want a
backportable fix..? I think this means we could return -EAGAIN from the
eofblocks ioctl, but afaict if something functionally conflicts with an
active scan across freeze then perhaps that's preferred.

Brian

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index a7be7a9e5c1a..0f14d58e5bb0 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -1515,13 +1515,24 @@ __xfs_icache_free_eofblocks(
 					   void *args),
 	int			tag)
 {
-	int flags = SYNC_TRYLOCK;
+	int			flags = SYNC_TRYLOCK;
+	int			error;
 
 	if (eofb && (eofb->eof_flags & XFS_EOF_FLAGS_SYNC))
 		flags = SYNC_WAIT;
 
-	return xfs_inode_ag_iterator_tag(mp, execute, flags,
-					 eofb, tag);
+	/*
+	 * freeze waits on background scanner jobs to complete so we cannot
+	 * block on write protection here. Bail if the transaction subsystem is
+	 * already freezing, returning -EAGAIN to notify other callers.
+	 */
+	if (!sb_start_write_trylock(mp->m_super))
+		return -EAGAIN;
+
+	error = xfs_inode_ag_iterator_tag(mp, execute, flags, eofb, tag);
+	sb_end_write(mp->m_super);
+
+	return error;
 }
 
 int

> --D
> 
> > Brian
> > 
> > > 
> > > Thanks,
> > > Paul
> > > 
> > > -- 
> > > You are receiving this mail because:
> > > You are watching the assignee of the bug.
> > > 
> > 
>