From: Hou Tao <[email protected]>
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 <[email protected]>
---
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
Hi,
Thanks for catching this.
On 11/28/22 11:19 AM, Hou Tao wrote:
> From: Hou Tao <[email protected]>
>
> 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 <[email protected]>
Reviewed-and-tested-by: Jingbo Xu <[email protected]>
> ---
> 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;
> }
> }
--
Thanks,
Jingbo
Hi David,
Could you please pick it up for v6.2 ?
On 11/28/2022 11:19 AM, Hou Tao wrote:
> From: Hou Tao <[email protected]>
>
> 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 <[email protected]>
> ---
> 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;
> }
> }
Hou Tao <[email protected]> wrote:
> > clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags);
Maybe this should be clear_bit_unlock() instead.
And I wonder if:
set_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &candidate->flags);
in fscache_hash_volume() needs a barrier before it.
> > - 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);
That doesn't seem right.
wake_up_bit() is more selective, so should be preferred to wake_up_var().
David
Hi David,
Sorry for the late reply. Busy for other business in work.
On 12/9/2022 7:26 PM, David Howells wrote:
> Hou Tao <[email protected]> wrote:
>
>>> clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags);
> Maybe this should be clear_bit_unlock() instead.
I'm not sure about that. In my understanding, clear_bit_unlock() is usually
paired with test_and_set_bit_lock() to implement bit lock to make sure the
writes before clear_bit_unlock() are visible to read access in concurrent
process, right ? But now the caller of fscache_wake_pending_volume() only modify
cursor->flags and nothing else, so I don't think it is needed here.
If its intended purpose is to provide the missing smp_mb() for wake_up_bit(), I
also don't think it is right, because the release barrier provided by
clear_bit_unlock() doesn't guarantee the order of cursor->flags and wq_head, so
I think one extra smp_mb_after_atomic() is also needed after
clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags).
If the above reasoning makes sense to you, I think we also need to add
smp_mb_after_atomic() for wake_up_bit() in fscache_create_volume_work().
> And I wonder if:
>
> set_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &candidate->flags);
>
> in fscache_hash_volume() needs a barrier before it.
I also don't get it. The barrier is used to guarantee the order between
cursor->flags and candidate->flags, right ? But the write and read of
cursor->flags and candidate->flags are protected by the same hash lock.
>
>>> - 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);
> That doesn't seem right.
>
> wake_up_bit() is more selective, so should be preferred to wake_up_var().
OK. Will update fscache_wait_on_volume_collision() to use wait_on_bit() accordingly.
> David
>
>
> .