Messages in this thread | | | Date | Tue, 27 Jan 2009 13:46:18 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code in print_fatal_signal() |
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* Oleg Nesterov <oleg@redhat.com> wrote:
> On 01/26, Ed Swierk wrote: > > > > On Tue, 2009-01-27 at 01:41 +0100, Oleg Nesterov wrote: > > > Ed, Ingo, but isn't it better to just use raw_smp_processor_id() in > > > __show_regs() ? This is only debug info, the printed CPU doesn't > > > have the "exact" meaning. > > > > I guess it doesn't really matter which CPU the signal handling thread > > happened to be running on, but are there other situations where > > show_regs() is always expected to print the correct CPU (and if not, > > why bother printing the CPU at all)? Disabling preemption here seems > > the safest approach and doesn't add much overhead. > > OK. > > > > And, without the comment, it is not easy to see why print_fatal_signal() > > > disables preeemption before show_regs(). > > > > Agreed; here's an updated patch. > > Actually, now I think show_regs() has other reasons to run with the > preemption disabled, __show_regs() does read_crX()/etc, I guess it is > better to stay on the same CPU throughout. > > So, Ed, I am sorry for noise.
another reason why it's good to run it with preemption disabled is that whatever context does show_regs() ought to be non-preemptible as it deals with CPU local details.
In the fatal-signals case we indeed have a "it does not really matter" boundary case, but in most of the other uses we want to be non-preemptible in debug contexts, and want a constant reminder in terms of smp_processor_id() warnings if that expectation is not met.
Ingo
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