[tip: smp/urgent] cpu: Fix broken cmdline "nosmp" and "maxcpus=0"

[tip-bot2 for Huacai Chen]

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

Commit-ID:     6ef8eb5125722c241fd60d7b0c872d5c2e5dd4ca
Gitweb:        https://git.kernel.org/tip/6ef8eb5125722c241fd60d7b0c872d5c2e5dd4ca
Author:        Huacai Chen <chenhuacai@xxxxxxxxxxx>
AuthorDate:    Tue, 18 Jun 2024 16:13:36 +08:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sun, 23 Jun 2024 20:04:14 +02:00

cpu: Fix broken cmdline "nosmp" and "maxcpus=0"

After the rework of "Parallel CPU bringup", the cmdline "nosmp" and
"maxcpus=0" parameters are not working anymore. These parameters set
setup_max_cpus to zero and that's handed to bringup_nonboot_cpus().

The code there does a decrement before checking for zero, which brings it
into the negative space and brings up all CPUs.

Add a zero check at the beginning of the function to prevent this.

[ tglx: Massaged change log ]

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@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20240618081336.3996825-1-chenhuacai@xxxxxxxxxxx

---
 kernel/cpu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 74cfdb6..3d2bf1d 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;