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Subject[PATCH v2] panic: Use atomic_try_cmpxchg in panic() and nmi_panic()
Date
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: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
v2: Replace semicolon with empty function body including comment.
---
kernel/panic.c | 22 +++++++++++++---------
1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 07239d4ad81e..9721492cf8a5 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)) {
+ /* go ahead */
+ } else if (old_cpu != this_cpu)
panic_smp_self_stop();

console_verbose();
--
2.41.0
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