[PATCH v2] smp: Use release stores for csd_lock_record() state

[Usama Arif <usama.arif@linux.dev>]

__csd_lock_record() publishes per-CPU CSD debug state that is read by
csd_lock_wait_toolong() on another CPU.  The remote side first reads
cur_csd with smp_load_acquire() and, when non-NULL, may then read the
matching cur_csd_func and cur_csd_info fields.

Use smp_store_release() when publishing cur_csd so that the preceding
cur_csd_func and cur_csd_info stores are ordered before the pointer
that csd_lock_wait_toolong() acquires.  This replaces the open-coded
smp_wmb() plus plain cur_csd store with the release operation that
matches the smp_load_acquire() in csd_lock_wait_toolong().

For the clear path, use smp_store_release(&cur_csd, NULL) so that
clearing the diagnostic state remains ordered after the preceding
callback/unlock work, without requiring a full barrier before the
store.  On x86 this removes the locked full barrier from the clear
path; on weaker memory models it uses the release operation needed by
the smp_load_acquire() in csd_lock_wait_toolong().

The old code also had smp_mb() calls around cur_csd updates. Those would
only be needed if cur_csd were treated as an exact live-state marker whose
publication had to be observed before callback execution or CSD unlock.
CSD stall warnings do not currently have RCU-style stall-ended checks, so
they already allow the stall to end while diagnostics are being assembled.
The cur_csd record is therefore best-effort diagnostic context, not a
precise completion/stall boundary.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
v1 -> v2: https://lore.kernel.org/all/01437928-ff79-4d8e-823b-7f20146946f6@linux.dev/
- Document where the smp_store_release() synchronizes with (Alan Stern,
  Randy Dunlap and Paul McKenney).
---
 kernel/smp.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index a0bb56bd8dda..685829875a3e 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)
-- 
2.53.0-Meta