Here are the threads I have:
{
pthread_rwlock_rdlock();
x = 1;
pthread_rwlock_rdunlock();
}
{
pthread_rwlock_wrlock();
if (x) {
x = 0;
} else {
foo();
}
pthread_rwlock_wrunlock();
}
Boehm's paper did not convince me of a bug.
Since pthread locks will include proper mfences I believe this to be correct
even on non-x86 machines.
Sent from my iPhone
On Dec 5, 2012, at 13:17, Brian Budge <[email protected]> wrote:
> It would be better to use atomics (with proper memory order), which can solve
> this kind of problem without unintended consequences. As mentioned before,
> this still may not be as maintenance friendly as locking, but at least it
> will be correct.
>
> Brian
>
> On Wed, Dec 5, 2012 at 9:29 AM, Julian Seward <[email protected]> wrote:
>>
>> On Tuesday, December 04, 2012, Patrick J. LoPresti wrote:
>> > There is no such thing as a benign data race. Ever.
>>
>> <soapbox>
>>
>> Well said. I couldn't have put any of this better myself.
>>
>> Having spent a considerable amount of time working on Helgrind and then
>> using it to chase races in some big hairy C++ codes, I became very
>> skeptical of the "oh it's only a harmless race" arguments. However,
>> I gave up shouting about it after a while since it just made me look
>> like a tiresome pedant hellbent on criticising people's clever go-faster-
>> by-avoiding-locking schemes.
>>
>> IMO, the idea that some races are harmless is a dangerous myth that
>> we should try to stamp out.
>>
>> I also noticed that threaded code that relies on unsynchronised
>> accesses is hard for newcomers to understand and reason about, so it
>> tends to be a maintenance hazard.
>>
>> In some ways, the fact that the Intel architecture guarantees to
>> deliver stores in-order (x86-TSO), and is therefore somewhat tolerant
>> of such racery, is a disadvantage. I think people would take this
>> stuff more seriously if racey code got trashed more often by machines
>> with memory systems that do reordering, such as Power7.
>>
>> </soapbox>
>>
>> J
>>
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