[PATCH v9 01/14] smp: Disable preemption explicitly in __csd_lock_wait()

[Chuyi Zhou]

The CSD lock wait debugging code in __csd_lock_wait() must run with
preemption disabled. The smp function call mechanisms which invoke it
currently keep preemption disabled across the wait, so the debugging code
inherits that guarantee from its callers.

Keeping preemption disabled across the whole smp function call operation
can induce large scheduling latencies. Shortening the caller-side
preemption-disabled region would invoke __csd_lock_wait() with preemption
enabled.

Prepare for that by disabling preemption explicitly around the CSD lock
wait debugging code in __csd_lock_wait().

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Muchun Song <muchun.song@xxxxxxxxx>
---
 kernel/smp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index a0bb56bd8dda..b58975480e11 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -323,6 +323,8 @@ static void __csd_lock_wait(call_single_data_t *csd)
 	int bug_id = 0;
 	u64 ts0, ts1;
 
+	guard(preempt)();
+
 	ts1 = ts0 = ktime_get_mono_fast_ns();
 	for (;;) {
 		if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id, &nmessages))
-- 
2.20.1