On 12/26/22 6:33 PM, Hou Tao wrote:
> 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
>
> Considering that wake_up_bit() is more selective, so fixing it by using
^
fix
> wait_on_bit() instead of wait_var_event() to wait for the freeing of
> relinquished volume. In addition because waitqueue_active() is used in
> wake_up_bit() and clear_bit() doesn't imply any memory barrier, so also
> adding smp_mb__after_atomic() before wake_up_bit().
... doesn't imply any memory barrier, add ...
>
> Fixes: 62ab63352350 ("fscache: Implement volume registration")
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
Otherwise LGTM :)
Reviewed-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> ---
> fs/fscache/volume.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
> index ab8ceddf9efa..fc3dd3bc851d 100644
> --- a/fs/fscache/volume.c
> +++ b/fs/fscache/volume.c
> @@ -141,13 +141,14 @@ static bool fscache_is_acquire_pending(struct fscache_volume *volume)
> static void fscache_wait_on_volume_collision(struct fscache_volume *candidate,
> unsigned int collidee_debug_id)
> {
> - wait_var_event_timeout(&candidate->flags,
> - !fscache_is_acquire_pending(candidate), 20 * HZ);
> + wait_on_bit_timeout(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
> + TASK_UNINTERRUPTIBLE, 20 * HZ);
> if (fscache_is_acquire_pending(candidate)) {
> pr_notice("Potential volume collision new=%08x old=%08x",
> candidate->debug_id, collidee_debug_id);
> fscache_stat(&fscache_n_volumes_collision);
> - wait_var_event(&candidate->flags, !fscache_is_acquire_pending(candidate));
> + wait_on_bit(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
> + TASK_UNINTERRUPTIBLE);
> }
> }
>
> @@ -348,6 +349,11 @@ 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);
> + /*
> + * Paired with barrier in wait_on_bit(). Check
> + * wake_up_bit() and waitqueue_active() for details.
> + */
> + smp_mb__after_atomic();
> wake_up_bit(&cursor->flags, FSCACHE_VOLUME_ACQUIRE_PENDING);
> return;
> }
--
Thanks,
Jingbo
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