padata: upgrade smp_mb__after_atomic to smp_mb in padata_do_serial

A 5.7 kernel hangs during a tcrypt test of padata that waits for an AEAD
request to finish.  This is only seen on large machines running many
concurrent requests.

The issue is that padata never serializes the request.  The removal of
the reorder_objects atomic missed that the memory barrier in
padata_do_serial() depends on it.

Upgrade the barrier from smp_mb__after_atomic to smp_mb to get correct
ordering again.

Fixes: 3facced7 ("padata: remove reorder_objects")
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

kernel/padata.c

@@ -335,7 +335,7 @@ static void padata_reorder(struct parallel_data *pd)
 	 *
 	 * Ensure reorder queue is read after pd->lock is dropped so we see
 	 * new objects from another task in padata_do_serial.  Pairs with
-	 * smp_mb__after_atomic in padata_do_serial.
+	 * smp_mb in padata_do_serial.
 	 */
 	smp_mb();
 
@@ -418,7 +418,7 @@ void padata_do_serial(struct padata_priv *padata)
 	 * with the trylock of pd->lock in padata_reorder.  Pairs with smp_mb
 	 * in padata_reorder.
 	 */
-	smp_mb__after_atomic();
+	smp_mb();
 
 	padata_reorder(pd);
 }