[PATCH] panic: Use atomic_try_cmpxchg in panic() and nmi_panic()

[Uros Bizjak]

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in panic() and nmi_panic().  x86 CMPXCHG instruction returns success in
ZF flag, so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, rename cpu variable to this_cpu in nmi_panic() and try to unify
logic flow between panic() and nmi_panic().

No functional change intended.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
 kernel/panic.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 07239d4ad81e..8740ac65cb2c 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -192,14 +192,15 @@ atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
  */
 void nmi_panic(struct pt_regs *regs, const char *msg)
 {
-	int old_cpu, cpu;
+	int old_cpu, this_cpu;
 
-	cpu = raw_smp_processor_id();
-	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, cpu);
+	old_cpu = PANIC_CPU_INVALID;
+	this_cpu = raw_smp_processor_id();
 
-	if (old_cpu == PANIC_CPU_INVALID)
+	/* atomic_try_cmpxchg updates old_cpu on failure */
+	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu))
 		panic("%s", msg);
-	else if (old_cpu != cpu)
+	else if (old_cpu != this_cpu)
 		nmi_panic_self_stop(regs);
 }
 EXPORT_SYMBOL(nmi_panic);
@@ -311,15 +312,18 @@ void panic(const char *fmt, ...)
 	 * stop themself or will wait until they are stopped by the 1st CPU
 	 * with smp_send_stop().
 	 *
-	 * `old_cpu == PANIC_CPU_INVALID' means this is the 1st CPU which
-	 * comes here, so go ahead.
+	 * cmpxchg success means this is the 1st CPU which comes here,
+	 * so go ahead.
 	 * `old_cpu == this_cpu' means we came from nmi_panic() which sets
 	 * panic_cpu to this CPU.  In this case, this is also the 1st CPU.
 	 */
+	old_cpu = PANIC_CPU_INVALID;
 	this_cpu = raw_smp_processor_id();
-	old_cpu  = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, this_cpu);
 
-	if (old_cpu != PANIC_CPU_INVALID && old_cpu != this_cpu)
+	/* atomic_try_cmpxchg updates old_cpu on failure */
+	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu))
+		;
+	else if (old_cpu != this_cpu)
 		panic_smp_self_stop();
 
 	console_verbose();
-- 
2.41.0