Re: [Xen-devel] memguard_guard_stack()

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 01.05.06 16:09 >>>
>
>On 1 May 2006, at 12:36, Jan Beulich wrote:
>
>> Hmm, but I found this precisely because we saw a double fault due to a 
>> stack overflow. Admittedly this was in the
>> context of one of these IPI storms during shotdown that were fixed 
>> previously, but even that shouldn't result in a stack
>> overflow, should it ? Jan
>
>How many CPUs were in the system? That code path was rather dodgy: the 
>function forcibly enabled interrupts and so a single CPU could nest in 
>that function up to (NR_CPUS-1) times which, if you had say 32 CPUs in 
>the system could certainly cause problems. I don't think it's 
>indicative of a wider problem in Xen -- for most interrupts (ones bound 
>to a guest) we don't even reenable interrupt delivery while handling 
>them, so nested ISRs in Xen are impossible.

At least 16 (both stack traces we got were on CPU 15.

>I would like to understand exactly what happened in the context of your 
>IPI storm (it *is* the machine restart bug we're talking about, right?) 

Yes.

>-- if you had much fewer than 32 CPUs then I need to check exactly how 
>much stack an invocation of machine_restart() uses.

A single nesting level consumed, according to the stack dump, up to 352 bytes.

Jan

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