[RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context

[Johannes Thumshirn <johannes.thumshirn@wdc.com>]

Fstests generic/113 exposes a deadlock introduced by the switch to iomap
for direct I/O.

[ 18.291293]
[ 18.291532] ============================================
[ 18.292115] WARNING: possible recursive locking detected
[ 18.292723] 5.9.0-rc2+ #746 Not tainted
[ 18.293145] --------------------------------------------
[ 18.293718] aio-stress/922 is trying to acquire lock:
[ 18.294274] ffff888217412010 (&sb->s_type->i_mutex_key#11){++++}-{3:3}, at: btrfs_sync_file+0xf7/0x560 [btrfs]
[ 18.295450]
[ 18.295450] but task is already holding lock:
[ 18.296086] ffff888217412010 (&sb->s_type->i_mutex_key#11){++++}-{3:3}, at: btrfs_file_write_iter+0x6e/0x630 [btrfs]
[ 18.297249]
[ 18.297249] other info that might help us debug this:
[ 18.297960] Possible unsafe locking scenario:
[ 18.297960]
[ 18.298605] CPU0
[ 18.298880] ----
[ 18.299151] lock(&sb->s_type->i_mutex_key#11);
[ 18.299653] lock(&sb->s_type->i_mutex_key#11);
[ 18.300156]
[ 18.300156] *** DEADLOCK ***
[ 18.300156]
[ 18.300802] May be due to missing lock nesting notation
[ 18.300802]
[ 18.301542] 2 locks held by aio-stress/922:
[ 18.302000] #0: ffff888217412010 (&sb->s_type->i_mutex_key#11){++++}-{3:3}, at: btrfs_file_write_iter+0x6e/0x630 [btrfs]
[ 18.303194] #1: ffff888217411ea0 (&ei->dio_sem){++++}-{3:3}, at: btrfs_direct_IO+0x113/0x160 [btrfs]
[ 18.304223]
[ 18.304223] stack backtrace:
[ 18.304695] CPU: 0 PID: 922 Comm: aio-stress Not tainted 5.9.0-rc2+ #746
[ 18.305383] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4-rebuilt.opensuse.org 04/01/2014
[ 18.306532] Call Trace:
[ 18.306796] dump_stack+0x78/0xa0
[ 18.307145] __lock_acquire.cold+0x121/0x29f
[ 18.307613] ? btrfs_dio_iomap_end+0x65/0x130 [btrfs]
[ 18.308140] lock_acquire+0x93/0x3b0
[ 18.308544] ? btrfs_sync_file+0xf7/0x560 [btrfs]
[ 18.309036] down_write+0x33/0x70
[ 18.309402] ? btrfs_sync_file+0xf7/0x560 [btrfs]
[ 18.309912] btrfs_sync_file+0xf7/0x560 [btrfs]
[ 18.310384] iomap_dio_complete+0x10d/0x120
[ 18.310824] iomap_dio_rw+0x3c8/0x520
[ 18.311225] btrfs_direct_IO+0xd3/0x160 [btrfs]
[ 18.311727] btrfs_file_write_iter+0x1fe/0x630 [btrfs]
[ 18.312264] ? find_held_lock+0x2b/0x80
[ 18.312662] aio_write+0xcd/0x180
[ 18.313011] ? __might_fault+0x31/0x80
[ 18.313408] ? find_held_lock+0x2b/0x80
[ 18.313817] ? __might_fault+0x31/0x80
[ 18.314217] io_submit_one+0x4e1/0xb30
[ 18.314606] ? find_held_lock+0x2b/0x80
[ 18.315010] __x64_sys_io_submit+0x71/0x220
[ 18.315449] do_syscall_64+0x33/0x40
[ 18.315829] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 18.316363] RIP: 0033:0x7f5940881f79
[ 18.316740] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d e7 4e 0c 00 f7 d8 64 89 01 48
[ 18.318651] RSP: 002b:00007f5934f51d88 EFLAGS: 00000246 ORIG_RAX: 00000000000000d1
[ 18.319428] RAX: ffffffffffffffda RBX: 00007f5934f52680 RCX: 00007f5940881f79
[ 18.320168] RDX: 0000000000b56030 RSI: 0000000000000008 RDI: 00007f593171f000
[ 18.320895] RBP: 00007f593171f000 R08: 0000000000000000 R09: 0000000000b56030
[ 18.321630] R10: 00007fffd599e080 R11: 0000000000000246 R12: 0000000000000008
[ 18.322369] R13: 0000000000000000 R14: 0000000000b56030 R15: 0000000000b56070

This happens because iomap_dio_complete() calls into generic_write_sync()
if we have the data-sync flag set. But as we're still under the
inode_lock() from btrfs_file_write_iter() we will deadlock once
btrfs_sync_file() tries to acquire the inode_lock().

Calling into generic_write_sync() is not needed as __btrfs_direct_write()
already takes care of persisting the data on disk. We can temporarily drop
the IOCB_DSYNC flag before calling into __btrfs_direct_write() so the
iomap code won't try to call into the sync routines as well.

References: https://github.com/btrfs/fstests/issues/12
Fixes: da4d7c1b4c45 ("btrfs: switch to iomap for direct IO")
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/file.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index b62679382799..c75c0f2a5f72 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2023,6 +2023,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 		atomic_inc(&BTRFS_I(inode)->sync_writers);
 
 	if (iocb->ki_flags & IOCB_DIRECT) {
+		iocb->ki_flags &= ~IOCB_DSYNC;
 		num_written = __btrfs_direct_write(iocb, from);
 	} else {
 		num_written = btrfs_buffered_write(iocb, from);
@@ -2046,8 +2047,10 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	if (num_written > 0)
 		num_written = generic_write_sync(iocb, num_written);
 
-	if (sync)
+	if (sync) {
+		iocb->ki_flags |= IOCB_DSYNC;
 		atomic_dec(&BTRFS_I(inode)->sync_writers);
+	}
 out:
 	current->backing_dev_info = NULL;
 	return num_written ? num_written : err;
-- 
2.26.2