On Fri, Feb 19, 2021 at 08:09:53AM -0500, Brian Foster wrote: > On Thu, Feb 18, 2021 at 08:29:40PM -0800, Darrick J. Wong wrote: > > From: Darrick J. Wong <djwong@xxxxxxxxxx> > > > > Brian Foster reported a lockdep warning on xfs/167: > > > > ============================================ > > WARNING: possible recursive locking detected > > 5.11.0-rc4 #35 Tainted: G W I > > -------------------------------------------- > > fsstress/17733 is trying to acquire lock: > > ffff8e0fd1d90650 (sb_internal){++++}-{0:0}, at: xfs_free_eofblocks+0x104/0x1d0 [xfs] > > > > but task is already holding lock: > > ffff8e0fd1d90650 (sb_internal){++++}-{0:0}, at: xfs_trans_alloc_inode+0x5f/0x160 [xfs] > > > > stack backtrace: > > CPU: 38 PID: 17733 Comm: fsstress Tainted: G W I 5.11.0-rc4 #35 > > Hardware name: Dell Inc. PowerEdge R740/01KPX8, BIOS 1.6.11 11/20/2018 > > Call Trace: > > dump_stack+0x8b/0xb0 > > __lock_acquire.cold+0x159/0x2ab > > lock_acquire+0x116/0x370 > > xfs_trans_alloc+0x1ad/0x310 [xfs] > > xfs_free_eofblocks+0x104/0x1d0 [xfs] > > xfs_blockgc_scan_inode+0x24/0x60 [xfs] > > xfs_inode_walk_ag+0x202/0x4b0 [xfs] > > xfs_inode_walk+0x66/0xc0 [xfs] > > xfs_trans_alloc+0x160/0x310 [xfs] > > xfs_trans_alloc_inode+0x5f/0x160 [xfs] > > xfs_alloc_file_space+0x105/0x300 [xfs] > > xfs_file_fallocate+0x270/0x460 [xfs] > > vfs_fallocate+0x14d/0x3d0 > > __x64_sys_fallocate+0x3e/0x70 > > do_syscall_64+0x33/0x40 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > The cause of this is the new code that spurs a scan to garbage collect > > speculative preallocations if we fail to reserve enough blocks while > > allocating a transaction. While the warning itself is a fairly benign > > lockdep complaint, it does bring to light a potential livelock. > > > > Specifically, when we kick off that scan, we're still holding onto the > > transaction's log reservation. If the blockgc scan finds something to > > free, it will need its own transaction, which means that it can block on > > the log grant. This means that if there are enough writer threads to > > take all the log reservation space with that first transaction, the > > second reservation attempts will all block on log space that cannot be > > freed, leading to a livelock. > > > > The text above around a prospective livelock doesn't seem accurate. > Otherwise the code looks fine to me. I don't have a preference between > this patch or the other one... Doh, that was leftover from when I thought there was a real log reservation deadlock opportunity. Will post v2. --D > Brian > > > Fix this by freeing the transaction and jumping back to xfs_trans_alloc > > like this patch in the V4 submission[1]. > > > > [1] https://lore.kernel.org/linux-xfs/161142798066.2171939.9311024588681972086.stgit@magnolia/ > > > > Fixes: a1a7d05a0576 ("xfs: flush speculative space allocations when we run out of space") > > Reported-by: Brian Foster <bfoster@xxxxxxxxxx> > > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx> > > --- > > fs/xfs/xfs_trans.c | 13 ++++++++++--- > > 1 file changed, 10 insertions(+), 3 deletions(-) > > > > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c > > index 44f72c09c203..377f3961d7ed 100644 > > --- a/fs/xfs/xfs_trans.c > > +++ b/fs/xfs/xfs_trans.c > > @@ -260,6 +260,7 @@ xfs_trans_alloc( > > struct xfs_trans **tpp) > > { > > struct xfs_trans *tp; > > + bool want_retry = true; > > int error; > > > > /* > > @@ -267,6 +268,7 @@ xfs_trans_alloc( > > * GFP_NOFS allocation context so that we avoid lockdep false positives > > * by doing GFP_KERNEL allocations inside sb_start_intwrite(). > > */ > > +retry: > > tp = kmem_cache_zalloc(xfs_trans_zone, GFP_KERNEL | __GFP_NOFAIL); > > if (!(flags & XFS_TRANS_NO_WRITECOUNT)) > > sb_start_intwrite(mp->m_super); > > @@ -289,7 +291,9 @@ xfs_trans_alloc( > > tp->t_firstblock = NULLFSBLOCK; > > > > error = xfs_trans_reserve(tp, resp, blocks, rtextents); > > - if (error == -ENOSPC) { > > + if (error == -ENOSPC && want_retry) { > > + xfs_trans_cancel(tp); > > + > > /* > > * We weren't able to reserve enough space for the transaction. > > * Flush the other speculative space allocations to free space. > > @@ -297,8 +301,11 @@ xfs_trans_alloc( > > * other locks. > > */ > > error = xfs_blockgc_free_space(mp, NULL); > > - if (!error) > > - error = xfs_trans_reserve(tp, resp, blocks, rtextents); > > + if (error) > > + return error; > > + > > + want_retry = false; > > + goto retry; > > } > > if (error) { > > xfs_trans_cancel(tp); > > >