Re: Xen PVM: Strange lockups when running PostgreSQL load

[Stefan Bader <stefan.bader@canonical.com>]

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On 19.10.2012 10:06, Jan Beulich wrote:
>>>> On 18.10.12 at 22:52, Stefan Bader <stefan.bader@canonical.com> wrote:
>> Actually I begin to suspect that it could be possible that I just overlooked 
>> the
>> most obvious thing. Provoking question: are we sure we are on the same page
>> about the purpose of the spin_lock_flags variant of the pv lock ops 
>> interface?
>>
>> I begin to suspect that it really is not for giving a chance to re-enable
>> interrupts. Just what it should be used for I am not clear. Anyway it seems 
>> all
>> other places more or less ignore the flags and map themselves back to an
>> ignorant version of spinlock.
>> Also I believe that the only high level function that would end up in 
>> passing
>> any flags, would be the spin_lock_irqsave one. And I am pretty sure that 
>> this
>> one will expect interrupts to stay disabled.
> 
> No - the only requirement here is that from the point on where
> the lock is owned interrupt must remain disabled. Re-enabling
> intermediately is quite okay (and used to be done by the
> native kernel prior to the conversion to ticket locks iirc).

Though it seems rather dangerous then. Don't remember the old code, but imo it
always opens up a (even microscopic) window to unexpected interruptions.

> 
>> So I tried below approach and that seems to be surviving the previously 
>> breaking
>> testcase for much longer than anything I tried before.
> 
> If that indeed fixes your problem, then (minus eventual problems
> with the scope of the interrupts-enabled window) this rather
> points at a bug in the users of the spinlock interfaces.

I would be pragmatic here, none of the other current implementations seem to
re-enable interrupts and so this only affects xen pv. And how much really is
gained from enabling it compared to the risk of being affected by something that
nobody else will be?

> 
> Jan
> 



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