[PATCH v3] smp: Use release stores for csd_lock_record() state

__csd_lock_record() publishes per-CPU diagnostic state (cur_csd,
cur_csd_func, cur_csd_info) that is consumed from a remote CPU by
csd_lock_wait_toolong() via smp_load_acquire(&cur_csd). To order the
matching cur_csd_func/cur_csd_info stores before the cur_csd
publication, the producer issues smp_wmb() before writing cur_csd;
to order the publication before the subsequent callback execution or
CSD unlock, it issues smp_mb() after the write. The clear path
mirrors this with smp_mb() before storing NULL into cur_csd so the
preceding callback/unlock is observed first.

The smp_mb() pair is heavier than what the consumer actually
requires (on x86 each emits a locked full barrier). The consumer
only needs to observe the matching cur_csd_func/cur_csd_info when it
sees a non-NULL cur_csd, and to observe the preceding callback/unlock
when it sees NULL -- both of which a release/acquire pair provides.
The extra two-way ordering enforced by smp_mb() -- that cur_csd
publication be observed before callback execution or unlock becomes
visible -- would only matter if cur_csd were an exact live-state
marker. csd_lock_wait_toolong() does not treat it that way: it
snapshots cur_csd via smp_load_acquire() and then prints / dumps /
re-IPIs without an RCU-style stall-ended recheck, so the diagnostic
already tolerates the remote CPU completing its work between snapshot
and report. cur_csd is best-effort context, not a precise stall
boundary.

Replace the smp_wmb() + plain store + smp_mb() in the publish path,
and the smp_mb() + plain store in the clear path, with
smp_store_release(). This pairs with the smp_load_acquire() in
csd_lock_wait_toolong(): preceding cur_csd_func/cur_csd_info stores
become visible before a remote reader observes the non-NULL
publication, and any preceding callback/unlock becomes visible before
a reader observes the NULL clear.

Signed-off-by: Usama Arif <[email protected]>
---
v2 -> v3: https://lore.kernel.org/all/[email protected]/
- Restructure changelog into context/problem/solution form (Thomas
  Gleixner).
- Add reciprocal pairing comment on the smp_load_acquire() in
  csd_lock_wait_toolong() (Dmitry).

v1 -> v2: https://lore.kernel.org/all/[email protected]/
- Document where the smp_store_release() synchronizes with (Alan
  Stern, Randy Dunlap and Paul McKenney).
---
 kernel/smp.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index a0bb56bd8dda..8a847a34f132 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -182,16 +182,22 @@ static atomic_t csd_bug_count = ATOMIC_INIT(0);
 static void __csd_lock_record(call_single_data_t *csd)
 {
 	if (!csd) {
-		smp_mb(); /* NULL cur_csd after unlock. */
-		__this_cpu_write(cur_csd, NULL);
+		/*
+		 * Pairs with smp_load_acquire() of cur_csd in
+		 * csd_lock_wait_toolong(): orders any preceding CSD
+		 * callback/unlock before a remote reader observes NULL.
+		 */
+		smp_store_release(this_cpu_ptr(&cur_csd), NULL);
 		return;
 	}
 	__this_cpu_write(cur_csd_func, csd->func);
 	__this_cpu_write(cur_csd_info, csd->info);
-	smp_wmb(); /* func and info before csd. */
-	__this_cpu_write(cur_csd, csd);
-	smp_mb(); /* Update cur_csd before function call. */
-		  /* Or before unlock, as the case may be. */
+	/*
+	 * Pairs with smp_load_acquire() of cur_csd in
+	 * csd_lock_wait_toolong(): publishes cur_csd_func and
+	 * cur_csd_info before the non-NULL pointer becomes visible.
+	 */
+	smp_store_release(this_cpu_ptr(&cur_csd), csd);
 }
 
 static __always_inline void csd_lock_record(call_single_data_t *csd)
@@ -272,7 +278,13 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 		cpux = 0;
 	else
 		cpux = cpu;
-	cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux)); /* Before func and info. */
+	/*
+	 * Pairs with smp_store_release() of cur_csd in __csd_lock_record():
+	 * a non-NULL cur_csd here implies cur_csd_func and cur_csd_info
+	 * are the matching publication; a NULL value is ordered after any
+	 * preceding CSD callback/unlock on the remote CPU.
+	 */
+	cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux));
 	/* How long since this CSD lock was stuck. */
 	ts_delta = ts2 - ts0;
 	pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %lld ns for CPU#%02d %pS(%ps).\n",
-- 
2.53.0-Meta