[PATCH v2 2/2] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted()

[Nathan Lynch <nathanl@linux.ibm.com>]

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 used speculatively, and the
function does not access per-cpu resources in a (Linux) preempt-unsafe way.
Use raw_smp_processor_id() to avoid such warnings, adding explanatory
comments.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()")
---
 arch/powerpc/include/asm/paravirt.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index 39f173961f6a..eb7df559ae74 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -110,7 +110,23 @@ 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;
+
+		/*
+		 * The result of vcpu_is_preempted() is used in a
+		 * speculative way, and is always subject to invalidation
+		 * by events internal and external to Linux. While we can
+		 * be called in preemptable context (in the Linux sense),
+		 * we're not accessing per-cpu resources in a way that can
+		 * race destructively with Linux scheduler preemption and
+		 * migration, and callers can tolerate the potential for
+		 * error introduced by sampling the CPU index without
+		 * pinning the task to it. So it is permissible to use
+		 * raw_smp_processor_id() here to defeat the preempt debug
+		 * warnings that can arise from using smp_processor_id()
+		 * in arbitrary contexts.
+		 */
+		first_cpu = cpu_first_thread_sibling(raw_smp_processor_id());
 
 		/*
 		 * The PowerVM hypervisor dispatches VMs on a whole core
-- 
2.31.1