[PATCH v3 0/2] Fixes for fscache volume operations

[PATCH v3 0/2] Fixes for fscache volume operations

[Hou Tao]

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset includes two fixes for fscache volume operations: patch 1
fixes the hang problem during volume acquisition when the volume
acquisition process waits for the freeing of relinquished volume, patch
2 adds the missing memory barrier in fscache_create_volume_work() and it
is spotted through code review when checking whether or not these is
missing smp_mb() before invoking wake_up_bit().

Comments are always welcome.

Chang Log:
v3:
 * Use clear_and_wake_up_bit() helper (Suggested by Jingbo Xu)
 * Tidy up commit message and add Reviewed-by tag

v2: https://listman.redhat.com/archives/linux-cachefs/2022-December/007402.html
 * rebased on v6.1-rc1
 * Patch 1: use wait_on_bit() instead (Suggested by David)
 * Patch 2: add the missing smp_mb() in fscache_create_volume_work()

v1: https://listman.redhat.com/archives/linux-cachefs/2022-December/007384.html


Hou Tao (2):
  fscache: Use wait_on_bit() to wait for the freeing of relinquished
    volume
  fscache: Use clear_and_wake_up_bit() in fscache_create_volume_work()

 fs/fscache/volume.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

-- 
2.29.2

[PATCH v3 1/2] fscache: Use wait_on_bit() to wait for the freeing of relinquished volume

[Hou Tao]

From: Hou Tao <houtao1@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

Considering that wake_up_bit() is more selective, so fix it by using
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, use
clear_and_wake_up_bit() to add the missing memory barrier between
cursor->flags and waitqueue_active().

Fixes: 62ab63352350 ("fscache: Implement volume registration")
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 fs/fscache/volume.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index ab8ceddf9efa..903af9d85f8b 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);
 	}
 }
 
@@ -347,8 +348,8 @@ static void fscache_wake_pending_volume(struct fscache_volume *volume,
 	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
 		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);
+			clear_and_wake_up_bit(FSCACHE_VOLUME_ACQUIRE_PENDING,
+					      &cursor->flags);
 			return;
 		}
 	}
-- 
2.29.2

Re: [PATCH v3 1/2] fscache: Use wait_on_bit() to wait for the freeing of relinquished volume

[Jeff Layton]

On Fri, 2023-01-13 at 19:52 +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@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
> 
> Considering that wake_up_bit() is more selective, so fix it by using
> 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, use
> clear_and_wake_up_bit() to add the missing memory barrier between
> cursor->flags and waitqueue_active().
> 
> Fixes: 62ab63352350 ("fscache: Implement volume registration")
> Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>  fs/fscache/volume.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
> index ab8ceddf9efa..903af9d85f8b 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);
>  	}
>  }
>  
> @@ -347,8 +348,8 @@ static void fscache_wake_pending_volume(struct fscache_volume *volume,
>  	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
>  		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);
> +			clear_and_wake_up_bit(FSCACHE_VOLUME_ACQUIRE_PENDING,
> +					      &cursor->flags);
>  			return;
>  		}
>  	}

Reviewed-by: Jeff Layton <jlayton@kernel.org>

[PATCH v3 2/2] fscache: Use clear_and_wake_up_bit() in fscache_create_volume_work()

[Hou Tao]

From: Hou Tao <houtao1@huawei.com>

fscache_create_volume_work() uses wake_up_bit() to wake up the processes
which are waiting for the completion of volume creation. According to
comments in wake_up_bit() and waitqueue_active(), an extra smp_mb() is
needed to guarantee the memory order between FSCACHE_VOLUME_CREATING
flag and waitqueue_active() before invoking wake_up_bit().

Fixing it by using clear_and_wake_up_bit() to add the missing memory
barrier.

Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 fs/fscache/volume.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index 903af9d85f8b..cdf991bdd9de 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -280,8 +280,7 @@ static void fscache_create_volume_work(struct work_struct *work)
 	fscache_end_cache_access(volume->cache,
 				 fscache_access_acquire_volume_end);
 
-	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
-	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
+	clear_and_wake_up_bit(FSCACHE_VOLUME_CREATING, &volume->flags);
 	fscache_put_volume(volume, fscache_volume_put_create_work);
 }
 
-- 
2.29.2

Re: [PATCH v3 2/2] fscache: Use clear_and_wake_up_bit() in fscache_create_volume_work()

[Jeff Layton]

On Fri, 2023-01-13 at 19:52 +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> fscache_create_volume_work() uses wake_up_bit() to wake up the processes
> which are waiting for the completion of volume creation. According to
> comments in wake_up_bit() and waitqueue_active(), an extra smp_mb() is
> needed to guarantee the memory order between FSCACHE_VOLUME_CREATING
> flag and waitqueue_active() before invoking wake_up_bit().
> 
> Fixing it by using clear_and_wake_up_bit() to add the missing memory
> barrier.
> 
> Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>  fs/fscache/volume.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
> index 903af9d85f8b..cdf991bdd9de 100644
> --- a/fs/fscache/volume.c
> +++ b/fs/fscache/volume.c
> @@ -280,8 +280,7 @@ static void fscache_create_volume_work(struct work_struct *work)
>  	fscache_end_cache_access(volume->cache,
>  				 fscache_access_acquire_volume_end);
>  
> -	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
> -	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
> +	clear_and_wake_up_bit(FSCACHE_VOLUME_CREATING, &volume->flags);
>  	fscache_put_volume(volume, fscache_volume_put_create_work);
>  }
>  

Reviewed-by: Jeff Layton <jlayton@kernel.org>

Re: [PATCH v3 0/2] Fixes for fscache volume operations

[Hou Tao]

Hi David,

Could you please pick it up for v6.2 ?

On 1/13/2023 7:52 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
>
> Hi,
>
> The patchset includes two fixes for fscache volume operations: patch 1
> fixes the hang problem during volume acquisition when the volume
> acquisition process waits for the freeing of relinquished volume, patch
> 2 adds the missing memory barrier in fscache_create_volume_work() and it
> is spotted through code review when checking whether or not these is
> missing smp_mb() before invoking wake_up_bit().
>
> Comments are always welcome.
>
> Chang Log:
> v3:
>  * Use clear_and_wake_up_bit() helper (Suggested by Jingbo Xu)
>  * Tidy up commit message and add Reviewed-by tag
>
> v2: https://listman.redhat.com/archives/linux-cachefs/2022-December/007402.html
>  * rebased on v6.1-rc1
>  * Patch 1: use wait_on_bit() instead (Suggested by David)
>  * Patch 2: add the missing smp_mb() in fscache_create_volume_work()
>
> v1: https://listman.redhat.com/archives/linux-cachefs/2022-December/007384.html
>
>
> Hou Tao (2):
>   fscache: Use wait_on_bit() to wait for the freeing of relinquished
>     volume
>   fscache: Use clear_and_wake_up_bit() in fscache_create_volume_work()
>
>  fs/fscache/volume.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>