netfs/fscache: Add a memory barrier for FSCACHE_VOLUME_CREATING

In fscache_create_volume(), there is a missing memory barrier between the
bit-clearing operation and the wake-up operation. This may cause a
situation where, after a wake-up, the bit-clearing operation hasn't been
detected yet, leading to an indefinite wait. The triggering process is as
follows:

  [cookie1]                [cookie2]                  [volume_work]
fscache_perform_lookup
  fscache_create_volume
                        fscache_perform_lookup
                          fscache_create_volume
			                        fscache_create_volume_work
                                                  cachefiles_acquire_volume
                                                  clear_and_wake_up_bit
    test_and_set_bit
                            test_and_set_bit
                              goto maybe_wait
      goto no_wait

In the above process, cookie1 and cookie2 has the same volume. When cookie1
enters the -no_wait- process, it will clear the bit and wake up the waiting
process. If a barrier is missing, it may cause cookie2 to remain in the
-wait- process indefinitely.

In commit 3288666 ("fscache: Use clear_and_wake_up_bit() in
fscache_create_volume_work()"), barriers were added to similar operations
in fscache_create_volume_work(), but fscache_create_volume() was missed.

By combining the clear and wake operations into clear_and_wake_up_bit() to
fix this issue.

Fixes: bfa22da ("fscache: Provide and use cache methods to lookup/create/free a volume")
Signed-off-by: Zizhi Wo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: David Howells <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Zizhi Wo

fs/netfs/fscache_volume.c

@@ -322,8 +322,7 @@ void fscache_create_volume(struct fscache_volume *volume, bool wait)
 	}
 	return;
 no_wait:
-	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);
 }
 
 /*