Re: [PATCH v2] futex: replace bare barrier() with a READ_ONCE()

[Darren Hart]

On Wed, Mar 02, 2016 at 09:08:31PM +0800, Jianyu Zhan wrote:

...

> 3) READ_ONCE() _might_ prevent more _theoretical_ "optimizations" by the compiler:
> 
>    The above retry logic is effectively the same as:
> 
> 	while (lock_ptr = READ_ONCE(q->lock_ptr)) {
>                    spin_lock(lock_ptr)


The spin_lock() is memory barrier, and therefor a general compiler barrier. The
READ_ONCE would be redundant in this case.

Unless you can demonstrate a failure mode in disassembly, or can point out how
the spin_lock barrier is insufficient that I have missed, this third point is
already covered.

-- 
Darren Hart
Intel Open Source Technology Center