[RFC PATCH] mm: Add missing release barrier on PGDAT_RECLAIM_LOCKED unlock

[Mathieu Desnoyers]

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 with following changes:

A) Use 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).

B) Use test_and_set_bit_lock rather than test_and_set_bit to test-and-set
   PGDAT_RECLAIM_LOCKED with an acquire memory ordering semantic.

This changes the "lock" acquisition from a full barrier to an acquire
memory ordering, which is weaker. The acquire semi-permeable barrier
paired with the release on unlock is sufficient for this mutual
exclusion use-case.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Jade Alglave <j.alglave@xxxxxxxxx>
Cc: Luc Maranget <luc.maranget@xxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c22175120f5d..021b25bdba91 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -7567,11 +7567,11 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
 	if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id != numa_node_id())
 		return NODE_RECLAIM_NOSCAN;
 
-	if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags))
+	if (test_and_set_bit_lock(PGDAT_RECLAIM_LOCKED, &pgdat->flags))
 		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_SUCCESS);
-- 
2.25.1