panic: use atomic_try_cmpxchg in panic() and nmi_panic()

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.

[ubizjak@gmail.com: clean up if/else block]
Link: https://lkml.kernel.org/r/20230906191200.68707-1-ubizjak@gmail.com
Link: https://lkml.kernel.org/r/20230904152230.9227-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/panic.c b/kernel/panic.c
index ffa037f..2807639 100644 (file)
--- 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();