mm: add missing release barrier on PGDAT_RECLAIM_LOCKED unlock

commit c0ebbb3841e07c4493e6fe351698806b09a87a37 upstream.

The PGDAT_RECLAIM_LOCKED bit is used to provide mutual exclusion of node
reclaim for struct pglist_data using a single bit.

It is "locked" with a test_and_set_bit (similarly to a try lock) which
provides full ordering with respect to loads and stores done within
__node_reclaim().

It is "unlocked" with clear_bit(), which does not provide any ordering
with respect to loads and stores done before clearing the bit.

The lack of clear_bit() memory ordering with respect to stores within
__node_reclaim() can cause a subsequent CPU to fail to observe stores from
a prior node reclaim.  This is not an issue in practice on TSO (e.g.
x86), but it is an issue on weakly-ordered architectures (e.g.  arm64).

Fix this by using clear_bit_unlock rather than clear_bit to clear
PGDAT_RECLAIM_LOCKED with a release memory ordering semantic.

This provides stronger memory ordering (release rather than relaxed).

Link: https://lkml.kernel.org/r/[email protected]
Fixes: d773ed6b85 ("mm: test and set zone reclaim lock before starting reclaim")
Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Andrea Parri <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: David Howells <[email protected]>
Cc: Jade Alglave <[email protected]>
Cc: Luc Maranget <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

mm/vmscan.c

@@ -4331,7 +4331,7 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
 		return NODE_RECLAIM_NOSCAN;
 
 	ret = __node_reclaim(pgdat, gfp_mask, order);
-	clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
+	clear_bit_unlock(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
 
 	if (!ret)
 		count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);