[PATCH] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted()

[Nathan Lynch]

vcpu_is_preempted() can be used outside of preempt-disabled critical
sections, yielding warnings such as:

BUG: using smp_processor_id() in preemptible [00000000] code: systemd-udevd/185
caller is rwsem_spin_on_owner+0x1cc/0x2d0
CPU: 1 PID: 185 Comm: systemd-udevd Not tainted 5.15.0-rc2+ #33
Call Trace:
[c000000012907ac0] [c000000000aa30a8] dump_stack_lvl+0xac/0x108 (unreliable)
[c000000012907b00] [c000000001371f70] check_preemption_disabled+0x150/0x160
[c000000012907b90] [c0000000001e0e8c] rwsem_spin_on_owner+0x1cc/0x2d0
[c000000012907be0] [c0000000001e1408] rwsem_down_write_slowpath+0x478/0x9a0
[c000000012907ca0] [c000000000576cf4] filename_create+0x94/0x1e0
[c000000012907d10] [c00000000057ac08] do_symlinkat+0x68/0x1a0
[c000000012907d70] [c00000000057ae18] sys_symlink+0x58/0x70
[c000000012907da0] [c00000000002e448] system_call_exception+0x198/0x3c0
[c000000012907e10] [c00000000000c54c] system_call_common+0xec/0x250

The result of vcpu_is_preempted() is always subject to invalidation by
events inside and outside of Linux; it's just a best guess at a point in
time. Use raw_smp_processor_id() to avoid such warnings.

Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()")
---
 arch/powerpc/include/asm/paravirt.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index bcb7b5f917be..e429aca566de 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -97,7 +97,14 @@ static inline bool vcpu_is_preempted(int cpu)
 
 #ifdef CONFIG_PPC_SPLPAR
 	if (!is_kvm_guest()) {
-		int first_cpu = cpu_first_thread_sibling(smp_processor_id());
+		int first_cpu;
+
+		/*
+		 * This is only a guess at best, and this function may be
+		 * called with preemption enabled. Using raw_smp_processor_id()
+		 * does not damage accuracy.
+		 */
+		first_cpu = cpu_first_thread_sibling(raw_smp_processor_id());
 
 		/*
 		 * Preemption can only happen at core granularity. This CPU
-- 
2.31.1