Re: Strange Windows problem, lock_timeout test request

2013-03-18 06:22 keltezéssel, Boszormenyi Zoltan írta:
> 2013-03-18 03:52 keltezéssel, Tom Lane írta:
>> Boszormenyi Zoltan <zb(at)cybertec(dot)at> writes:
>>> 2013-03-17 16:07 keltezéssel, Tom Lane írta:
>>>> It suddenly occurs to me though that there's more than one way to skin
>>>> this cat. We could easily add another static flag variable called
>>>> "sigalrm_allowed" or some such, and have the signal handler test that
>>>> and immediately return without doing anything if it's off. Clearing
>>>> and setting such a variable would be a lot cheaper than an extra
>>>> setitimer call, as well as more robust since it would protect us from
>>>> all sources of SIGALRM not just ITIMER_REAL.
>>> Something like the attached patch?
>> Well, something like that, but it still needed some improvements. In
>> particular I thought it best to leave the signal handler still releasing
>> the procLatch unconditionally --- that behavior is independent of what
>> this module is doing. Also you seem to have some odd ideas about what
>> do-while will accomplish. AFAIK, in this usage it's purely a syntactic
>> trick without much semantic content. It's the marking of the variable
>> as "volatile" that counts for telling the compiler not to re-order
>> operations.
>
> The volatile marking shouldn't even be necessary there.
> The signal handler doesn't writes to it, only the main code.
> I just put it there saying "why not?" to myself.
> IIRC, volatile is needed if both the signal handler and the
> main code changes the same variable.

Also, since the the variable assignment doesn't depend on other code
in the same function (or vice-versa) the compiler is still free to
reorder it.

Volatile is about not caching the variable in a CPU register since
it's "volatile"...

>
> I got the reordering idea from here:
> http://yarchive.net/comp/linux/compiler_barriers.html
>
> Thanks for committing,
> Zoltán Böszörményi
>

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