[PATCH] cpumask: avoid WARN in prefill_possible_map()

[Dmitry Safonov <dsafonov@virtuozzo.com>]

With CONFIG_DEBUG_PER_CPU_MAPS and CONFIG_CPUMASK_OFFSTACK enabled
fixes the following WARN_ON_ONCE() for booting with nr_cpus=1:

[    0.000000] Linux version 4.9.0 (dsafonov@localhost.localdomain) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC) ) #36 SMP Mon Dec 12 18:05:46 MSK 2016
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.9.0 root=/dev/mapper/vz-root ro crashkernel=auto rd.lvm.lv=vz/root rd.lvm.lv=vz/swap console=ttyS0,115200 vsyscall=none nr_cpus=1
[    0.000000] smpboot: 4 Processors exceeds NR_CPUS limit of 1
[    0.000000] smpboot: Allowing 1 CPUs, 0 hotplug CPUs
[    0.000000] ------------[ cut here ]------------
[    0.000000] WARNING: CPU: 0 PID: 0 at ./include/linux/cpumask.h:121 cpumask_check.part.2+0x1c/0x1e
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.9.0 #36
[    0.000000] Call Trace:
[    0.000000]  [<ffffffff8136664b>] dump_stack+0x67/0x9c
[    0.000000]  [<ffffffff81063e21>] __warn+0xd1/0xf0
[    0.000000]  [<ffffffff81063f0d>] warn_slowpath_null+0x1d/0x20
[    0.000000]  [<ffffffff8105f4ca>] cpumask_check.part.2+0x1c/0x1e
[    0.000000]  [<ffffffff8104008e>] cpumask_clear_cpu+0x2e/0x40
[    0.000000]  [<ffffffff81f88a62>] prefill_possible_map+0x15c/0x16a
[    0.000000]  [<ffffffff81f802c6>] setup_arch+0xba7/0xc33
[    0.000000]  [<ffffffff81f78c59>] start_kernel+0x63/0x448
[    0.000000]  [<ffffffff81f7858c>] x86_64_start_reservations+0x2a/0x2c
[    0.000000]  [<ffffffff81f78678>] x86_64_start_kernel+0xea/0xed
[    0.000000] ---[ end trace 5876da8d2ace83fb ]---

nr_cpu_ids is set to possible two lines futher - omit checking in
set_cpu_possible() cycles.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: x86@kernel.org
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
---
 arch/x86/kernel/smpboot.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 42f5eb7b4f6c..17167bec7c61 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1459,6 +1459,9 @@ __init void prefill_possible_map(void)
 	pr_info("Allowing %d CPUs, %d hotplug CPUs\n",
 		possible, max_t(int, possible - num_processors, 0));
 
+	/* Avoid WARN() in set_cpu_possible()=>cpumask_check() */
+	nr_cpu_ids = NR_CPUS;
+
 	for (i = 0; i < possible; i++)
 		set_cpu_possible(i, true);
 	for (; i < NR_CPUS; i++)
-- 
2.10.2