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

[Michael Ellerman]

Nathan Lynch <nathanl at linux.ibm.com> writes:
> 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());

This change seems good, except I think the comment needs to be a lot
more explicit about what it's doing and why.

A casual reader is going to be confused about vcpu preemption vs
"preemption", which are basically unrelated yet use the same word.

It's not clear how raw_smp_processor_id() is related to (Linux)
preemption, unless you know that smp_processor_id() is the alternative
and it contains a preemption check.

And "this is only a guess" is not clear on what *this* is, you're
referring to the result of the whole function, but that's not obvious.

>  		/*
>  		 * Preemption can only happen at core granularity. This CPU
                   ^^^^^^^^^^
                   Means something different to "preemption" above.

I know you didn't write that comment, and maybe we need to rewrite some
of those existing comments to make it clear they're not talking about
Linux preemption.

cheers