[PATCH 2/2] xfs_repair: retain superblock buffer to avoid write hook deadlock

Darrick J. Wong
2022-08-09T21:08:16Z

From: Darrick J. Wong <djwong@kernel.org>

Every now and then I experience the following deadlock in xfs_repair
when I'm running the offline repair fuzz tests:

#0 futex_wait (private=0, expected=2, futex_word=0x55555566df70) at ../sysdeps/nptl/futex-internal.h:146
#1 __GI___lll_lock_wait (futex=futex@entry=0x55555566df70, private=0) at ./nptl/lowlevellock.c:49
#2 lll_mutex_lock_optimized (mutex=0x55555566df70) at ./nptl/pthread_mutex_lock.c:48
#3 ___pthread_mutex_lock (mutex=mutex@entry=0x55555566df70) at ./nptl/pthread_mutex_lock.c:93
#4 cache_shake (cache=cache@entry=0x55555566de60, priority=priority@entry=2, purge=purge@entry=false) at cache.c:231
#5 cache_node_get (cache=cache@entry=0x55555566de60, key=key@entry=0x7fffe55e01b0, nodep=nodep@entry=0x7fffe55e0168) at cache.c:452
#6 __cache_lookup (key=key@entry=0x7fffe55e01b0, flags=0, bpp=bpp@entry=0x7fffe55e0228) at rdwr.c:405
#7 libxfs_getbuf_flags (btp=0x55555566de00, blkno=0, len=<optimized out>, flags=<optimized out>, bpp=0x7fffe55e0228) at rdwr.c:457
#8 libxfs_buf_read_map (btp=0x55555566de00, map=map@entry=0x7fffe55e0280, nmaps=nmaps@entry=1, flags=flags@entry=0, bpp=bpp@entry=0x7fffe55e0278, ops=0x5555556233e0 <xfs_sb_buf_ops>)
at rdwr.c:704
#9 libxfs_buf_read (ops=<optimized out>, bpp=0x7fffe55e0278, flags=0, numblks=<optimized out>, blkno=0, target=<optimized out>)
at /storage/home/djwong/cdev/work/xfsprogs/build-x86_64/libxfs/libxfs_io.h:195
#10 libxfs_getsb (mp=mp@entry=0x7fffffffd690) at rdwr.c:162
#11 force_needsrepair (mp=0x7fffffffd690) at xfs_repair.c:924
#12 repair_capture_writeback (bp=<optimized out>) at xfs_repair.c:1000
#13 libxfs_bwrite (bp=0x7fffe011e530) at rdwr.c:869
#14 cache_shake (cache=cache@entry=0x55555566de60, priority=priority@entry=2, purge=purge@entry=false) at cache.c:240
#15 cache_node_get (cache=cache@entry=0x55555566de60, key=key@entry=0x7fffe55e0470, nodep=nodep@entry=0x7fffe55e0428) at cache.c:452
#16 __cache_lookup (key=key@entry=0x7fffe55e0470, flags=1, bpp=bpp@entry=0x7fffe55e0538) at rdwr.c:405
#17 libxfs_getbuf_flags (btp=0x55555566de00, blkno=12736, len=<optimized out>, flags=<optimized out>, bpp=0x7fffe55e0538) at rdwr.c:457
#18 __libxfs_buf_get_map (btp=<optimized out>, map=map@entry=0x7fffe55e05b0, nmaps=<optimized out>, flags=flags@entry=1, bpp=bpp@entry=0x7fffe55e0538) at rdwr.c:501
#19 libxfs_buf_get_map (btp=<optimized out>, map=map@entry=0x7fffe55e05b0, nmaps=<optimized out>, flags=flags@entry=1, bpp=bpp@entry=0x7fffe55e0538) at rdwr.c:525
#20 pf_queue_io (args=args@entry=0x5555556722c0, map=map@entry=0x7fffe55e05b0, nmaps=<optimized out>, flag=flag@entry=11) at prefetch.c:124
#21 pf_read_bmbt_reclist (args=0x5555556722c0, rp=<optimized out>, numrecs=78) at prefetch.c:220
#22 pf_scan_lbtree (dbno=dbno@entry=1211, level=level@entry=1, isadir=isadir@entry=1, args=args@entry=0x5555556722c0, func=0x55555557f240 <pf_scanfunc_bmap>) at prefetch.c:298
#23 pf_read_btinode (isadir=1, dino=<optimized out>, args=0x5555556722c0) at prefetch.c:385
#24 pf_read_inode_dirs (args=args@entry=0x5555556722c0, bp=bp@entry=0x7fffdc023790) at prefetch.c:459
#25 pf_read_inode_dirs (bp=<optimized out>, args=0x5555556722c0) at prefetch.c:411
#26 pf_batch_read (args=args@entry=0x5555556722c0, which=which@entry=PF_PRIMARY, buf=buf@entry=0x7fffd001d000) at prefetch.c:609
#27 pf_io_worker (param=0x5555556722c0) at prefetch.c:673
#28 start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#29 clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

From this stack trace, we see that xfs_repair's prefetch module is
getting some xfs_buf objects ahead of initiating a read (#19). The
buffer cache has hit its limit, so it calls cache_shake (#14) to free
some unused xfs_bufs. The buffer it finds is a dirty buffer, so it
calls libxfs_bwrite to flush it out to disk, which in turn invokes the
buffer write hook that xfs_repair set up in 3b7667cb to mark the ondisk
filesystem's superblock as NEEDSREPAIR until repair actually completes.

Unfortunately, the NEEDSREPAIR handler itself needs to grab the
superblock buffer, so it makes another call into the buffer cache (#9),
which sees that the cache is full and tries to shake it(#4). Hence we
deadlock on cm_mutex because shaking is not reentrant.

Fix this by retaining a reference to the superblock buffer when possible
so that the writeback hook doesn't have to access the buffer cache to
set NEEDSREPAIR.

Fixes: 3b7667cb ("xfs_repair: set NEEDSREPAIR the first time we write to a filesystem")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
libxfs/libxfs_api_defs.h | 2 +
libxfs/libxfs_io.h | 1 +
libxfs/rdwr.c | 8 +++++
repair/phase2.c | 8 +++++
repair/protos.h | 1 +
repair/xfs_repair.c | 75 ++++++++++++++++++++++++++++++++++++++++------
6 files changed, 86 insertions(+), 9 deletions(-)

diff --git a/libxfs/libxfs_api_defs.h b/libxfs/libxfs_api_defs.h
index 824f2c4d..7c59bd4b 100644
--- a/libxfs/libxfs_api_defs.h
+++ b/libxfs/libxfs_api_defs.h
@@ -53,9 +53,11 @@
#define xfs_buf_delwri_submit libxfs_buf_delwri_submit
#define xfs_buf_get libxfs_buf_get
#define xfs_buf_get_uncached libxfs_buf_get_uncached
+#define xfs_buf_lock libxfs_buf_lock
#define xfs_buf_read libxfs_buf_read
#define xfs_buf_read_uncached libxfs_buf_read_uncached
#define xfs_buf_relse libxfs_buf_relse
+#define xfs_buf_unlock libxfs_buf_unlock
#define xfs_bunmapi libxfs_bunmapi
#define xfs_bwrite libxfs_bwrite
#define xfs_calc_dquots_per_chunk libxfs_calc_dquots_per_chunk
diff --git a/libxfs/libxfs_io.h b/libxfs/libxfs_io.h
index 9c0e2704..fae86427 100644
--- a/libxfs/libxfs_io.h
+++ b/libxfs/libxfs_io.h
@@ -226,6 +226,7 @@ xfs_buf_hold(struct xfs_buf *bp)
}

void xfs_buf_lock(struct xfs_buf *bp);
+void xfs_buf_unlock(struct xfs_buf *bp);

int libxfs_buf_get_uncached(struct xfs_buftarg *targ, size_t bblen, int flags,
struct xfs_buf **bpp);
diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index fe69f9b4..61083dac 100644
--- a/libxfs/rdwr.c
+++ b/libxfs/rdwr.c
@@ -384,6 +384,14 @@ xfs_buf_lock(
pthread_mutex_lock(&bp->b_lock);
}

+void
+xfs_buf_unlock(
+ struct xfs_buf *bp)
+{
+ if (use_xfs_buf_lock)
+ pthread_mutex_unlock(&bp->b_lock);
+}
+
static int
__cache_lookup(
struct xfs_bufkey *key,
diff --git a/repair/phase2.c b/repair/phase2.c
index 56a39bb4..2ada95ae 100644
--- a/repair/phase2.c
+++ b/repair/phase2.c
@@ -370,6 +370,14 @@ phase2(
} else
do_log(_("Phase 2 - using internal log\n"));

+ /*
+ * Now that we've set up the buffer cache the way we want it, try to
+ * grab our own reference to the primary sb so that the hooks will not
+ * have to call out to the buffer cache.
+ */
+ if (mp->m_buf_writeback_fn)
+ retain_primary_sb(mp);
+
/* Zero log if applicable */
do_log(_(" - zero log...\n"));

diff --git a/repair/protos.h b/repair/protos.h
index 03ebae14..83e471ff 100644
--- a/repair/protos.h
+++ b/repair/protos.h
@@ -16,6 +16,7 @@ int get_sb(xfs_sb_t *sbp,
xfs_off_t off,
int size,
xfs_agnumber_t agno);
+int retain_primary_sb(struct xfs_mount *mp);
void write_primary_sb(xfs_sb_t *sbp,
int size);

diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
index c94671d8..d46870cc 100644
--- a/repair/xfs_repair.c
+++ b/repair/xfs_repair.c
@@ -749,6 +749,63 @@ check_fs_vs_host_sectsize(
}
}

+/*
+ * If we set up a writeback function to set NEEDSREPAIR while the filesystem is
+ * dirty, there's a chance that calling libxfs_getsb could deadlock the buffer
+ * cache while trying to get the primary sb buffer if the first non-sb write to
+ * the filesystem is the result of a cache shake. Retain a reference to the
+ * primary sb buffer to avoid all that.
+ */
+static struct xfs_buf *primary_sb_bp; /* buffer for superblock */
+
+int
+retain_primary_sb(
+ struct xfs_mount *mp)
+{
+ int error;
+
+ error = -libxfs_buf_read(mp->m_ddev_targp, XFS_SB_DADDR,
+ XFS_FSS_TO_BB(mp, 1), 0, &primary_sb_bp,
+ &xfs_sb_buf_ops);
+ if (error)
+ return error;
+
+ libxfs_buf_unlock(primary_sb_bp);
+ return 0;
+}
+
+static void
+drop_primary_sb(void)
+{
+ if (!primary_sb_bp)
+ return;
+
+ libxfs_buf_lock(primary_sb_bp);
+ libxfs_buf_relse(primary_sb_bp);
+ primary_sb_bp = NULL;
+}
+
+static int
+get_primary_sb(
+ struct xfs_mount *mp,
+ struct xfs_buf **bpp)
+{
+ int error;
+
+ *bpp = NULL;
+
+ if (!primary_sb_bp) {
+ error = retain_primary_sb(mp);
+ if (error)
+ return error;
+ }
+
+ libxfs_buf_lock(primary_sb_bp);
+ xfs_buf_hold(primary_sb_bp);
+ *bpp = primary_sb_bp;
+ return 0;
+}
+
/* Clear needsrepair after a successful repair run. */
void
clear_needsrepair(
@@ -769,15 +826,14 @@ clear_needsrepair(
do_warn(
_("Cannot clear needsrepair due to flush failure, err=%d.\n"),
error);
- return;
+ goto drop;
}

/* Clear needsrepair from the superblock. */
- bp = libxfs_getsb(mp);
- if (!bp || bp->b_error) {
+ error = get_primary_sb(mp, &bp);
+ if (error) {
do_warn(
- _("Cannot clear needsrepair from primary super, err=%d.\n"),
- bp ? bp->b_error : ENOMEM);
+ _("Cannot clear needsrepair from primary super, err=%d.\n"), error);
} else {
mp->m_sb.sb_features_incompat &=
~XFS_SB_FEAT_INCOMPAT_NEEDSREPAIR;
@@ -786,6 +842,8 @@ clear_needsrepair(
}
if (bp)
libxfs_buf_relse(bp);
+drop:
+ drop_primary_sb();
}

static void
@@ -808,11 +866,10 @@ force_needsrepair(
xfs_sb_version_needsrepair(&mp->m_sb))
return;

- bp = libxfs_getsb(mp);
- if (!bp || bp->b_error) {
+ error = get_primary_sb(mp, &bp);
+ if (error) {
do_log(
- _("couldn't get superblock to set needsrepair, err=%d\n"),
- bp ? bp->b_error : ENOMEM);
+ _("couldn't get superblock to set needsrepair, err=%d\n"), error);
} else {
/*
* It's possible that we need to set NEEDSREPAIR before we've