* Nathan Lynch [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 [] 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:
> [c00012907ac0] [c0aa30a8] dump_stack_lvl+0xac/0x108 (unreliable)
> [c00012907b00] [c1371f70] check_preemption_disabled+0x150/0x160
> [c00012907b90] [c01e0e8c] rwsem_spin_on_owner+0x1cc/0x2d0
> [c00012907be0] [c01e1408] rwsem_down_write_slowpath+0x478/0x9a0
> [c00012907ca0] [c0576cf4] filename_create+0x94/0x1e0
> [c00012907d10] [c057ac08] do_symlinkat+0x68/0x1a0
> [c00012907d70] [c057ae18] sys_symlink+0x58/0x70
> [c00012907da0] [c002e448] system_call_exception+0x198/0x3c0
> [c00012907e10] [c000c54c] 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
> Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in
> vcpu_is_preempted()")
Looks good to me.
Reviewed-by: Srikar Dronamraju
> ---
> 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
