The patch titled
Subject: smpboot: use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death
has been added to the -mm mm-nonmm-unstable branch. Its filename is
smpboot-use-atomic_try_cmpxchg-in-cpu_wait_death-and-cpu_report_death.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/smpboot-use-atomic_try_cmpxchg-in-cpu_wait_death-and-cpu_report_death.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: smpboot: use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death
Date: Tue, 23 Aug 2022 17:13:36 +0200
Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in cpu_wait_death and cpu_report_death. 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, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.
No functional change intended.
Link: https://lkml.kernel.org/r/20220823151336.3984-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/smpboot.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
--- a/kernel/smpboot.c~smpboot-use-atomic_try_cmpxchg-in-cpu_wait_death-and-cpu_report_death
+++ a/kernel/smpboot.c
@@ -444,16 +444,16 @@ bool cpu_wait_death(unsigned int cpu, in
break;
sleep_jf = DIV_ROUND_UP(sleep_jf * 11, 10);
}
-update_state:
oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
+update_state:
if (oldstate == CPU_DEAD) {
/* Outgoing CPU died normally, update state. */
smp_mb(); /* atomic_read() before update. */
atomic_set(&per_cpu(cpu_hotplug_state, cpu), CPU_POST_DEAD);
} else {
/* Outgoing CPU still hasn't died, set state accordingly. */
- if (atomic_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
- oldstate, CPU_BROKEN) != oldstate)
+ if (!atomic_try_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
+ &oldstate, CPU_BROKEN))
goto update_state;
ret = false;
}
@@ -475,14 +475,14 @@ bool cpu_report_death(void)
int newstate;
int cpu = smp_processor_id();
+ oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
do {
- oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
if (oldstate != CPU_BROKEN)
newstate = CPU_DEAD;
else
newstate = CPU_DEAD_FROZEN;
- } while (atomic_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
- oldstate, newstate) != oldstate);
+ } while (!atomic_try_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
+ &oldstate, newstate));
return newstate == CPU_DEAD;
}
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
epoll-use-try_cmpxchg-in-list_add_tail_lockless.patch
buffer-use-try_cmpxchg-in-discard_buffer.patch
aio-use-atomic_try_cmpxchg-in-__get_reqs_available.patch
iversion-use-atomic64_try_cmpxchg.patch
bitops-use-try_cmpxchg-in-set_mask_bits-and-bit_clear_unless.patch
task_work-use-try_cmpxchg-in-task_work_add-task_work_cancel_match-and-task_work_run.patch
smpboot-use-atomic_try_cmpxchg-in-cpu_wait_death-and-cpu_report_death.patch
