Hi,
On 1/12/2023 11:47 AM, Jingbo Xu wrote:
>
> 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 ...
Thanks for suggestions above. Will update in v3.
>
>> Fixes: 62ab63352350 ("fscache: Implement volume registration")
>> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
>
> Otherwise LGTM :)
>
> Reviewed-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
Thanks for review.
>
>> ---
>> 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;
>> }
--
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