Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

[Peter Zijlstra]

On Wed, Aug 31, 2016 at 07:28:18AM +1000, Benjamin Herrenschmidt wrote:
> It's always been a requirement that if you actually context switch a
> full mb() is implied ...

> On powerpc we have a sync deep in _switch to achieve that.

OK, fair enough. I must've missed it in the x86 switch_to, must be one
of those implied serializing instructions I'm not too familiar with.

> (though that isn't the case if you don't actually
> switch, ie, you are back to RUNNING before you even hit schedule).

Right, which invalidates the claim that schedule() implies a full mb,

> This is necessary so that a process who wakes up on a different CPU sees
> all of its own load/stores.

Don't actually think its needed for that, see the comment from
8643cda549ca4, the scheduler has enough barriers to guarantee
Program-Order for tasks without that.