| Message ID | 20190515150317.1833-1-fdmanana@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/3] Btrfs: fix fsync not persisting changed attributes of a directory | expand |
On Wed, May 15, 2019 at 04:03:17PM +0100, fdmanana@kernel.org wrote: > From: Filipe Manana <fdmanana@suse.com> > > When syncing the log, the final phase of a fsync operation, we need to > either create a log root's item or update the existing item in the log > tree of log roots, and that depends on the current value of the log > root's log_transid - if it's 1 we need to create the log root item, > otherwise it must exist already and we update it. Since there is no > synchronization between updating the log_transid and checking it for > deciding whether the log root's item needs to be created or updated, we > end up with a tiny race window that results in attempts to update the > item to fail because the item was not yet created: > > CPU 1 CPU 2 > > btrfs_sync_log() > > lock root->log_mutex > > set log root's log_transid to 1 > > unlock root->log_mutex > > btrfs_sync_log() > > lock root->log_mutex > > sets log root's > log_transid to 2 > > unlock root->log_mutex > > update_log_root() > > sees log root's log_transid > with a value of 2 > > calls btrfs_update_root(), > which fails with -EUCLEAN > and causes transaction abort > > Until recently the race lead to a BUG_ON at btrfs_update_root(), but after > the recent commit 7ac1e464c4d47 ("btrfs: Don't panic when we can't find a > root key") we just abort the current transaction. > > A sample trace of the BUG_ON() on a SLE12 kernel: > > ------------[ cut here ]------------ > kernel BUG at ../fs/btrfs/root-tree.c:157! > Oops: Exception in kernel mode, sig: 5 [#1] > SMP NR_CPUS=2048 NUMA pSeries > (...) > Supported: Yes, External > CPU: 78 PID: 76303 Comm: rtas_errd Tainted: G X 4.4.156-94.57-default #1 > task: c00000ffa906d010 ti: c00000ff42b08000 task.ti: c00000ff42b08000 > NIP: d000000036ae5cdc LR: d000000036ae5cd8 CTR: 0000000000000000 > REGS: c00000ff42b0b860 TRAP: 0700 Tainted: G X (4.4.156-94.57-default) > MSR: 8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 22444484 XER: 20000000 > CFAR: d000000036aba66c SOFTE: 1 > GPR00: d000000036ae5cd8 c00000ff42b0bae0 d000000036bda220 0000000000000054 > GPR04: 0000000000000001 0000000000000000 c00007ffff8d37c8 0000000000000000 > GPR08: c000000000e19c00 0000000000000000 0000000000000000 3736343438312079 > GPR12: 3930373337303434 c000000007a3a800 00000000007fffff 0000000000000023 > GPR16: c00000ffa9d26028 c00000ffa9d261f8 0000000000000010 c00000ffa9d2ab28 > GPR20: c00000ff42b0bc48 0000000000000001 c00000ff9f0d9888 0000000000000001 > GPR24: c00000ffa9d26000 c00000ffa9d261e8 c00000ffa9d2a800 c00000ff9f0d9888 > GPR28: c00000ffa9d26028 c00000ffa9d2aa98 0000000000000001 c00000ffa98f5b20 > NIP [d000000036ae5cdc] btrfs_update_root+0x25c/0x4e0 [btrfs] > LR [d000000036ae5cd8] btrfs_update_root+0x258/0x4e0 [btrfs] > Call Trace: > [c00000ff42b0bae0] [d000000036ae5cd8] btrfs_update_root+0x258/0x4e0 [btrfs] (unreliable) > [c00000ff42b0bba0] [d000000036b53610] btrfs_sync_log+0x2d0/0xc60 [btrfs] > [c00000ff42b0bce0] [d000000036b1785c] btrfs_sync_file+0x44c/0x4e0 [btrfs] > [c00000ff42b0bd80] [c00000000032e300] vfs_fsync_range+0x70/0x120 > [c00000ff42b0bdd0] [c00000000032e44c] do_fsync+0x5c/0xb0 > [c00000ff42b0be10] [c00000000032e8dc] SyS_fdatasync+0x2c/0x40 > [c00000ff42b0be30] [c000000000009488] system_call+0x3c/0x100 > Instruction dump: > 7f43d378 4bffebb9 60000000 88d90008 3d220000 e8b90000 3b390009 e87a01f0 > e8898e08 e8f90000 4bfd48e5 60000000 <0fe00000> e95b0060 39200004 394a0ea0 > ---[ end trace 8f2dc8f919cabab8 ]--- > > So fix this by doing the check of log_transid and updating or creating the > log root's item while holding the root's log_mutex. > > Fixes: 7237f1833601d ("Btrfs: fix tree logs parallel sync") > Signed-off-by: Filipe Manana <fdmanana@suse.com> Added to 5.2-rc queue, thanks.
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index 7d13533a9620..6391df35ca48 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -3096,6 +3096,12 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans, log->log_transid = root->log_transid; root->log_start_pid = 0; /* + * Update or create log root item under the root's log_mutex to prevent + * races with concurrent log syncs that can lead to failure to update + * log root item because it was not created yet. + */ + ret = update_log_root(trans, log); + /* * IO has been started, blocks of the log tree have WRITTEN flag set * in their headers. new modifications of the log will be written to * new positions. so it's safe to allow log writers to go in. @@ -3114,8 +3120,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans, mutex_unlock(&log_root_tree->log_mutex); - ret = update_log_root(trans, log); - mutex_lock(&log_root_tree->log_mutex); if (atomic_dec_and_test(&log_root_tree->log_writers)) { /* atomic_dec_and_test implies a barrier */