[PATCH 4/4] powerpc/pseries: warn if recursing into the hcall tracing code
npiggin at gmail.com
Tue May 4 20:45:34 AEST 2021
Excerpts from Naveen N. Rao's message of May 4, 2021 8:25 pm:
> Nicholas Piggin wrote:
>> Excerpts from Naveen N. Rao's message of April 27, 2021 11:59 pm:
>>> Nicholas Piggin wrote:
>>>> + *
>>>> + * H_CONFER from spin locks must be treated separately though and use _notrace
>>>> + * plpar_hcall variants, see yield_to_preempted().
>>>> static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
>>>> @@ -1843,7 +1846,7 @@ notrace void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
>>>> depth = this_cpu_ptr(&hcall_trace_depth);
>>>> - if (*depth)
>>>> + if (WARN_ON_ONCE(*depth))
>>>> goto out;
>>> I don't think this will be helpful. The hcall trace depth tracking is
>>> for the tracepoint and I suspect that this warning will be triggered
>>> quite easily. Since we have recursion protection, I don't think we
>>> should warn here.
>> What would trigger recursion?
> The trace code that this protects: trace_hcall_entry(). The tracing code
> itself can end up doing a hcall as we see in the first patch in this
> plpar_hcall_norets_trace+0x34/0x8c (unreliable)
> There is also a comment aroung hcall_trace_depth that mentions this:
> * Since the tracing code might execute hcalls we need to guard against
> * recursion. One example of this are spinlocks calling H_YIELD on
> * shared processor partitions.
Right but since fixing those, my thought is we better not cause more
any recursion, so we should fix anything that does.
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