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

Commit-ID:     21782b3a5cd40892cb2995aa1ec3e74dd1112f1d
Gitweb:        https://git.kernel.org/tip/21782b3a5cd40892cb2995aa1ec3e74dd1112f1d
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Sat, 22 Nov 2025 16:19:18 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 23 Nov 2025 19:59:30 +01:00

cpu: Initialize __num_possible_cpus correctly

The variable to cache the number of possible CPUs is initialized to NR_CPUS
at build time, but that's only correct when cpu_possible_mask is
initialized with CPU_BITS_ALL. That's only the case on PARISC.

On x86 and some other architectures this does not matter because they
initialize cpu_possible_mask via init_cpu_possible() which does a proper
weight calculation. Though on architectures which do not, this results
in a completely wrong cached value 'NR_CPUS + actual possible CPUs'.

Initialize it correctly to 0 when CONFIG_INIT_ALL_POSSIBLE=n and move the
NR_CPUS initialization into the PARISC specific section.

Fixes: d0f23ccf6ba9 ("cpumask: Cache num_possible_cpus()")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/87zf8ehyf7.ffs@tglx
Closes: https://lore.kernel.org/all/89c7106e-a431-443a-9527-3d5fbce77fe1@samsung.com
Closes: https://lore.kernel.org/all/20251122002755.GA2682494@ax162
Closes: https://lore.kernel.org/all/ad2a55f9-9bca-41bf-a6ec-efb23eaa778f@paulmck-laptop
---
 kernel/cpu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 18e530d..b674fdf 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3085,10 +3085,13 @@ EXPORT_SYMBOL(cpu_all_bits);
 #ifdef CONFIG_INIT_ALL_POSSIBLE
 struct cpumask __cpu_possible_mask __ro_after_init
 	= {CPU_BITS_ALL};
+unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
 #else
 struct cpumask __cpu_possible_mask __ro_after_init;
+unsigned int __num_possible_cpus __ro_after_init;
 #endif
 EXPORT_SYMBOL(__cpu_possible_mask);
+EXPORT_SYMBOL(__num_possible_cpus);
 
 struct cpumask __cpu_online_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_online_mask);
@@ -3108,9 +3111,6 @@ EXPORT_SYMBOL(__cpu_dying_mask);
 atomic_t __num_online_cpus __read_mostly;
 EXPORT_SYMBOL(__num_online_cpus);
 
-unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
-EXPORT_SYMBOL(__num_possible_cpus);
-
 void init_cpu_present(const struct cpumask *src)
 {
 	cpumask_copy(&__cpu_present_mask, src);