linux-kernel - [PATCH v2] smpboot: Use atomic_try_cmpxchg in cpu_wait_death and cpu_report

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Message-Id: <20220825145603.5811-1-ubizjak@gmail.com>
Date:   Thu, 25 Aug 2022 16:56:03 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, kernel test robot <lkp@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v2] smpboot: Use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death

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.

Reported-by: kernel test robot <lkp@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
Changes since v1:
- Fix uninitialized variable in case cpu dies early.
---
 kernel/smpboot.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index b9f54544e749..2c7396da470c 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -433,7 +433,7 @@ bool cpu_wait_death(unsigned int cpu, int seconds)
 
 	/* The outgoing CPU will normally get done quite quickly. */
 	if (atomic_read(&per_cpu(cpu_hotplug_state, cpu)) == CPU_DEAD)
-		goto update_state;
+		goto update_state_early;
 	udelay(5);
 
 	/* But if the outgoing CPU dawdles, wait increasingly long times. */
@@ -444,16 +444,17 @@ bool cpu_wait_death(unsigned int cpu, int seconds)
 			break;
 		sleep_jf = DIV_ROUND_UP(sleep_jf * 11, 10);
 	}
-update_state:
+update_state_early:
 	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 +476,14 @@ 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;
 }
 
-- 
2.37.1