linux-kernel - [PATCH] cpu: Fix broken cmdline "nosmp" and "maxcpus=0"

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Date: Tue, 18 Jun 2024 16:13:36 +0800
From: Huacai Chen <chenhuacai@...ngson.cn>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Xuefeng Li <lixuefeng@...ngson.cn>,
	Huacai Chen <chenhuacai@...il.com>,
	Jiaxun Yang <jiaxun.yang@...goat.com>,
	Huacai Chen <chenhuacai@...ngson.cn>,
	stable@...r.kernel.org
Subject: [PATCH] cpu: Fix broken cmdline "nosmp" and "maxcpus=0"

After the reworking of "Parallel CPU bringup", the cmdline "nosmp" and
"maxcpus=0" is broken. Because these parameters make setup_max_cpus be
zero, and setup_max_cpus is the "max_cpus" of bringup_nonboot_cpus().
In this case, "if (!--ncpus)" will not be true in cpuhp_bringup_mask(),
and the result is all the possible cpus are brought up.

We can fix it by changing "if (!--ncpus)" to "if (!ncpus--)". But to
make logic more clear and save some cpu cycles, it is better to check
"max_cpus" in bringup_nonboot_cpus(), return early if it is zero.

Cc: stable@...r.kernel.org
Fixes: 18415f33e2ac4ab382 ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE")
Fixes: 06c6796e0304234da6 ("cpu/hotplug: Fix off by one in cpuhp_bringup_mask()")
Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
---
 kernel/cpu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 563877d6c28b..200974a31de8 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1859,6 +1859,9 @@ static inline bool cpuhp_bringup_cpus_parallel(unsigned int ncpus) { return fals

 void __init bringup_nonboot_cpus(unsigned int max_cpus)
 {
+	if (!max_cpus)
+		return;
+
 	/* Try parallel bringup optimization if enabled */
 	if (cpuhp_bringup_cpus_parallel(max_cpus))
 		return;
-- 
2.43.0