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

[Hou Tao]

Hi David,

Could you please pick it up for v6.2 ?

On 11/28/2022 11:19 AM, Hou Tao wrote:
> From: Hou Tao <houtao1 at huawei.com>
>
> 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 at huawei.com>
> ---
>  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;
>  		}
>  	}