Messages in this thread |  | | | Date | Tue, 19 Jan 2010 09:19:43 +0800 | | From | Xiao Guangrong <> | | Subject | Re: [PATCH 1/3] perf_event: fix race in perf_swevent_get_recursion_context() |
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Frederic Weisbecker wrote: > On Mon, Jan 18, 2010 at 09:42:34PM +0800, Xiao Guangrong wrote: >> It only disable preemption in perf_swevent_get_recursion_context() >> it can't avoid race of hard-irq and NMI >> >> In this patch, we use atomic operation to avoid it and reduce >> cpu_ctx->recursion size, it also make this patch no need diable >> preemption >> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> > > > > I don't understand what is racy in what we have currently. >
It's because hard-irq(we can handle interruption with interruption enabled) and NMI are nested, for example:
int perf_swevent_get_recursion_context(void) { ...... if (cpuctx->recursion[rctx]) { put_cpu_var(perf_cpu_context); return -1; } /* * Another interruption handler/NMI will re-enter there if it * happed, it make the recursion value chaotic */ cpuctx->recursion[rctx]++; ...... } > > >> int perf_swevent_get_recursion_context(void) >> { >> - struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context); >> + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context); >> int rctx; >> >> if (in_nmi()) >> @@ -3933,13 +3933,8 @@ int perf_swevent_get_recursion_context(void) >> else >> rctx = 0; >> >> - if (cpuctx->recursion[rctx]) { >> - put_cpu_var(perf_cpu_context); >> + if (test_and_set_bit(rctx, &cpuctx->recursion)) >> return -1; > > > > This looks broken. We don't call back perf_swevent_put_recursion_context > in fail case, so the bit won't ever be cleared once we recurse. >
Um, i think we can't clear the bit in this fail case, consider below sequence:
path A: path B
set bit but find the bit already set atomic set bit | | | V | handle SW event | | V V exit and not clear the bit atomic clear bit After A and B, the bit is still zero
Right? :-)
Thanks, Xiao
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