The patch titled
Subject: panic: use atomic_try_cmpxchg in panic() and nmi_panic()
has been added to the -mm mm-nonmm-unstable branch. Its filename is
panic-use-atomic_try_cmpxchg-in-panic-and-nmi_panic.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/panic-use-atomic_try_cmpxchg-in-panic-and-nmi_panic.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: panic: use atomic_try_cmpxchg in panic() and nmi_panic()
Date: Mon, 4 Sep 2023 17:21:01 +0200
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.
Link: https://lkml.kernel.org/r/20230904152230.9227-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/panic.c | 22 +++++++++++++---------
1 file changed, 13 insertions(+), 9 deletions(-)
--- a/kernel/panic.c~panic-use-atomic_try_cmpxchg-in-panic-and-nmi_panic
+++ a/kernel/panic.c
@@ -192,14 +192,15 @@ atomic_t panic_cpu = ATOMIC_INIT(PANIC_C
*/
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();
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
panic-use-atomic_try_cmpxchg-in-panic-and-nmi_panic.patch
