Re: [RFC/SERIOUS] grilling troubled CPUs for fun and profit?

[John Richard Moser]

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Andreas Mohr wrote:
> Hello all,
> 
> while looking for loop places to apply cpu_relax() to, I found the
> following gems:
> 
> arch/i386/kernel/crash.c/crash_nmi_callback():
> 
>         /* Assume hlt works */
>         halt();
>         for(;;);
> 
>         return 1;
> }
> 
> arch/i386/kernel/doublefault.c/doublefault_fn():
> 
>         for (;;) /* nothing */;
> }
> 
> Let's assume that we have a less than moderate fan failure that causes
> the CPU to heat up beyond the critical limit...
> That might result in - you guessed it - crashes or doublefaults.
> In which case we enter the corresponding handler and do... what?

Looks like it calls halt() to put the CPU into idle mode, and then
performs a nop?  (I think the null condition evaluates false.... not
sure, haven't tried this before!)

> Exactly, we accelerate the CPUs happy march into bit heaven by letting it
> execute a busy-loop under a non-working fan.
> Thanks, your users will be very happy, I think ;)
> (especially since it was "just" a simple fan failure that could have been
> entirely remedied by buying another fan for $3)
> 
> 
> The same thing applies to
> arch/i386/kernel/smp.c/stop_this_cpu(), albeit there it's less catastrophic
> due to most likely normal working conditions there.
> 
> IMHO on any critical CPU failure we should:
> - try to log it (might be difficult with a broken CPU, though)
> - optionally somehow directly alert the user
> - STOP the system, COMPLETELY (that way people WILL take notice, hopefully
>   before it's too late and actual damage will have occurred)
> - make DAMN SURE that the (possibly already broken) CPU won't have a
>   less than nice time once the system is stopped
> 
> Am I completely missing something here?
> 
> If this is an issue, then maybe we should consolidate those places into
> one function that safely(!) halts a CPU, optionally disabling APIC etc.
> 
> Oh, and once you finished processing my mail here, you could optionally
> also look at my report about almost unusably broken USB:
> http://lkml.org/lkml/2006/6/19/54
> (no replies yet despite advanced breakage)
> 
> Thanks!
> 
> Andreas Mohr
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