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

From: Srikar Dronamraju <[email-masked]>
Date: Wed, 29 Sep 2021 13:00:36 +0530

* Nathan Lynch <[email-masked]> [2021-09-28 16:41:47]:

> 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 <[email-masked]>
> Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()")

Looks good to me.

Reviewed-by: Srikar Dronamraju <[email-masked]>

> ---
>  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
> 

-- 
Thanks and Regards
Srikar Dronamraju