[PATCH] fscache: Use wake_up_var() to wake up pending volume acquisition

[Hou Tao]

From: Hou Tao <houtao1@xxxxxxxxxx>

The freeing of relinquished volume will wake up the pending volume
acquisition by using wake_up_bit(), however it is mismatched with
wait_var_event() used in fscache_wait_on_volume_collision() and it will
never wake up the waiter in the wait-queue because these two functions
operate on different wait-queues.

According to the implementation in fscache_wait_on_volume_collision(),
if the wake-up of pending acquisition is delayed longer than 20 seconds
(e.g., due to the delay of on-demand fd closing), the first
wait_var_event_timeout() will timeout and the following wait_var_event()
will hang forever as shown below:

 FS-Cache: Potential volume collision new=00000024 old=00000022
 ......
 INFO: task mount:1148 blocked for more than 122 seconds.
       Not tainted 6.1.0-rc6+ #1
 task:mount           state:D stack:0     pid:1148  ppid:1
 Call Trace:
  <TASK>
  __schedule+0x2f6/0xb80
  schedule+0x67/0xe0
  fscache_wait_on_volume_collision.cold+0x80/0x82
  __fscache_acquire_volume+0x40d/0x4e0
  erofs_fscache_register_volume+0x51/0xe0 [erofs]
  erofs_fscache_register_fs+0x19c/0x240 [erofs]
  erofs_fc_fill_super+0x746/0xaf0 [erofs]
  vfs_get_super+0x7d/0x100
  get_tree_nodev+0x16/0x20
  erofs_fc_get_tree+0x20/0x30 [erofs]
  vfs_get_tree+0x24/0xb0
  path_mount+0x2fa/0xa90
  do_mount+0x7c/0xa0
  __x64_sys_mount+0x8b/0xe0
  do_syscall_64+0x30/0x60
  entry_SYSCALL_64_after_hwframe+0x46/0xb0

Fixing it by using wake_up_var() instead of wake_up_bit(). In addition
because waitqueue_active() is used in wake_up_var() and clear_bit()
doesn't imply any memory barrier, so do smp_mb__after_atomic() before
invoking wake_up_var().

Fixes: 62ab63352350 ("fscache: Implement volume registration")
Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
---
 fs/fscache/volume.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index ab8ceddf9efa..cf8293bb1aca 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -348,7 +348,12 @@ static void fscache_wake_pending_volume(struct fscache_volume *volume,
 		if (fscache_volume_same(cursor, volume)) {
 			fscache_see_volume(cursor, fscache_volume_see_hash_wake);
 			clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags);
-			wake_up_bit(&cursor->flags, FSCACHE_VOLUME_ACQUIRE_PENDING);
+			/*
+			 * Paired with barrier in wait_var_event(). Check
+			 * waitqueue_active() and wake_up_var() for details.
+			 */
+			smp_mb__after_atomic();
+			wake_up_var(&cursor->flags);
 			return;
 		}
 	}
-- 
2.29.2