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

[Srikar Dronamraju]

* Michael Ellerman <mpe at ellerman.id.au> [2021-09-23 17:29:32]:

> Nathan Lynch <nathanl at linux.ibm.com> writes:
> > Srikar Dronamraju <srikar at linux.vnet.ibm.com> writes:
> >
> >> * Nathan Lynch <nathanl at linux.ibm.com> [2021-09-22 11:01:12]:
> >>
> >>> Srikar Dronamraju <srikar at linux.vnet.ibm.com> writes:
> >>> > * Nathan Lynch <nathanl at linux.ibm.com> [2021-09-20 22:12:13]:
> >>> >
> >>> >> 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.
> >>> >
> >>> > Typically smp_processor_id() and raw_smp_processor_id() except for the
> >>> > CONFIG_DEBUG_PREEMPT.
> >>> 
> >>> Sorry, I don't follow...
> >>
> >> I meant, Unless CONFIG_DEBUG_PREEMPT, smp_processor_id() is defined as
> >> raw_processor_id().
> >>
> >>> 
> >>> > In the CONFIG_DEBUG_PREEMPT case, smp_processor_id()
> >>> > is actually debug_smp_processor_id(), which does all the checks.
> >>> 
> >>> Yes, OK.
> >>> 
> >>> > I believe these checks in debug_smp_processor_id() are only valid for x86
> >>> > case (aka cases were they have __smp_processor_id() defined.)
> >>> 
> >>> Hmm, I am under the impression that the checks in
> >>> debug_smp_processor_id() are valid regardless of whether the arch
> >>> overrides __smp_processor_id().
> >>
> >> From include/linux/smp.h
> >>
> >> /*
> >>  * Allow the architecture to differentiate between a stable and unstable read.
> >>  * For example, x86 uses an IRQ-safe asm-volatile read for the unstable but a
> >>  * regular asm read for the stable.
> >>  */
> >> #ifndef __smp_processor_id
> >> #define __smp_processor_id(x) raw_smp_processor_id(x)
> >> #endif
> >>
> >> As far as I see, only x86 has a definition of __smp_processor_id.
> >> So for archs like Powerpc, __smp_processor_id(), is always
> >> defined as raw_smp_processor_id(). Right?
> >
> > Sure, yes.
> >
> >> I would think debug_smp_processor_id() would be useful if __smp_processor_id()
> >> is different from raw_smp_processor_id(). Do note debug_smp_processor_id() 
> >> calls raw_smp_processor_id().
> 
> Agree.
> 
> > I do not think the utility of debug_smp_processor_id() is related to
> > whether the arch defines __smp_processor_id().
> >
> >> Or can I understand how debug_smp_processor_id() is useful if
> >> __smp_processor_id() is defined as raw_smp_processor_id()?
> 
> debug_smp_processor_id() is useful on powerpc, as well as other arches,
> because it checks that we're in a context where the processor id won't
> change out from under us.
> 
> eg. something like this is unsafe:
> 
>   int counts[NR_CPUS];
>   int tmp, cpu;
>   
>   cpu = smp_processor_id();
>   tmp = counts[cpu];
>   				<- preempted here and migrated to another CPU
>   counts[cpu] = tmp + 1;
> 

If lets say the above call was replaced by raw_smp_processor_id(), how would
it avoid the preemption / migration to another CPU?

Replacing it with raw_smp_processor_id() may avoid, the debug splat but the
underlying issue would still remain as is. No?

> 
> > So, for powerpc with DEBUG_PREEMPT unset, a call to smp_procesor_id()
> > expands to __smp_processor_id() which expands to raw_smp_processor_id(),
> > avoiding the preempt safety checks. This is working as intended.
> >
> > For powerpc with DEBUG_PREEMPT=y, a call to smp_processor_id() expands
> > to the out of line call to debug_smp_processor_id(), which calls
> > raw_smp_processor_id() and performs the checks, warning if called in an
> > inappropriate context, as seen here. Also working as intended.
> >
> > AFAICT __smp_processor_id() is a performance/codegen-oriented hook, and
> > not really related to the debug facility. Please see 9ed7d75b2f09d
> > ("x86/percpu: Relax smp_processor_id()").
> 
> Yeah good find.
> 
> cheers

-- 
Thanks and Regards
Srikar Dronamraju