On Mon, Oct 23, 2017 at 10:36:21PM -0300, James Almer wrote: > On 10/23/2017 10:24 PM, Michael Niedermayer wrote: > > On Tue, Oct 24, 2017 at 01:39:03AM +0100, Mark Thompson wrote: > >> On 24/10/17 00:52, Michael Niedermayer wrote: > >>> Hi > >>> > >>> On Mon, Oct 23, 2017 at 04:43:19PM +0100, Mark Thompson wrote: > >>>> On 23/10/17 03:56, Michael Niedermayer wrote: > >>>>> the initialization should be thread safe as it never writes a different > >>>>> value in the same spot > >>>> > >>>> This is not true; please be very careful with assumptions like this. > >>>> > >>>> The C standard calls this a data race and it is undefined behaviour. > >>> > >>> Thats interresting, we definitly do not want undefined behavior > >>> can you quote the relevant parts of the C standard > >>> which make writing the same value, a data race ? > >>> > >>> I was not aware of this if its true. > >>> Also i dont immedeatly see this in the "latest" draft i am looking at > >>> atm > >>> > >>> What i see is this: > >>> 4 Two expression evaluations conflict if one of them modifies a memory > >>> location and the > >>> other one reads or modifies the same memory location. > >>> ... > >>> 25 The execution of a program contains a data race if it contains two > >>> conflicting actions in > >>> different threads, at least one of which is not atomic, and neither > >>> happens before the > >>> other. Any such data race results in undefined behavior. > >>> > >>> This seems carefully worded to not include writing the same value. > >>> As "modification" implies change which does not occur when writing the > >>> same value. > >> > >> All writes are modifications. > >> > >> See section 3.1 note 2: > >> """ > >> 3 NOTE 2 "Modify"€™ includes the case where the new value being stored > >> is the same as the previous value. > >> """ > > > > That resolves the difference between our interpretation > > > > thanks > > Should i push this, or does someone want to give ff_thread_once() a try?
If ff_thread_once() is used outside init code, it should be benchmarked as it may be a bad idea speed wise. During init speed doesnt matter so ff_thread_once() there would be fine but would require more changes to move it to init. I have no oppinion about ff_thread_once() vs hardcoding ATM thx [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB The greatest way to live with honor in this world is to be what we pretend to be. -- Socrates
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