linux-kernel - [tip: smp/core] cpu/hotplug: Fix off by one in cpuhp_bringup

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Message-ID: <168485843734.404.16847199234383553669.tip-bot2@tip-bot2>
Date:   Tue, 23 May 2023 16:13:57 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Mark Brown <broonie@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: smp/core] cpu/hotplug: Fix off by one in cpuhp_bringup_mask()

The following commit has been merged into the smp/core branch of tip:

Commit-ID:     06c6796e0304234da65e70577f354cb194086521
Gitweb:        https://git.kernel.org/tip/06c6796e0304234da65e70577f354cb194086521
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Tue, 23 May 2023 01:12:26 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Tue, 23 May 2023 18:06:40 +02:00

cpu/hotplug: Fix off by one in cpuhp_bringup_mask()

cpuhp_bringup_mask() iterates over a cpumask and starts all present CPUs up
to a caller provided upper limit.

The limit variable is decremented and checked for 0 before invoking
cpu_up(), which is obviously off by one and prevents the bringup of the
last CPU when the limit is equal to the number of present CPUs.

Move the decrement and check after the cpu_up() invocation.

Fixes: 18415f33e2ac ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE")
Reported-by: Mark Brown <broonie@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Mark Brown <broonie@...nel.org>
Link: https://lore.kernel.org/r/87wn10ufj9.ffs@tglx
---
 kernel/cpu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 005f863..88a7ede 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1770,9 +1770,6 @@ static void __init cpuhp_bringup_mask(const struct cpumask *mask, unsigned int n
 	for_each_cpu(cpu, mask) {
 		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 
-		if (!--ncpus)
-			break;
-
 		if (cpu_up(cpu, target) && can_rollback_cpu(st)) {
 			/*
 			 * If this failed then cpu_up() might have only
@@ -1781,6 +1778,9 @@ static void __init cpuhp_bringup_mask(const struct cpumask *mask, unsigned int n
 			 */
 			WARN_ON(cpuhp_invoke_callback_range(false, cpu, st, CPUHP_OFFLINE));
 		}
+
+		if (!--ncpus)
+			break;
 	}
 }