Re: (ITS#7969) LMDB: Globally shared fields of meta-data are not 'volatile'.

[h.b.furuseth@usit.uio.no]

On 13/01/15 17:54, leo@yuriev.ru wrote:
> 2015-01-13 13:50 GMT+03:00 Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>:
>> comp.programming.threads's advise is that volatile is useless for thread
>> sync:  You need sync primitives, and then you don't need volatile since
>> the compiler/hardware may not move the memory access past the primitive.
>
> Excuse me, but seems you are misunderstand comp.programming.threads in
> case of blocking/locking multithreading and non-blocking
> multithreading (aka lock-free/wait-free).

Yes, that didn't come out right.  I don't mind inserting the
volatile, but I don't know that it helps either.  As far as I
understand if it was broken without volatile, then it's still
broken with it - just hopefully less likely to break.  And LMDB
couldn't be releaseed with your original unportable sync code
along with the volatile.

> (...)
>> Unless the primitive's spec says so, I guess.  E.g. with an "asm"
>> statement where the compiler does not know what the assembly code does
>> and must be told not to mess with ordering.
>
> Please study GCC's manual about __asm __volalile (:::"memory"), this
> is just a "full compiler fence".

Thanks, looks good.  Will read the rest later.

-- 
Hallvard