Re: [patch 20/24] x86/speculation: Split out TIF update

[Thomas Gleixner]

On Tue, 27 Nov 2018, Tim Chen wrote:
> On 11/27/2018 02:36 PM, Thomas Gleixner wrote:
> >> We need this special handling only if the next task has TIF_SPEC_UPDATE 
> >> set, which is one-off event globally (when seccomp marks all its threads 
> >> so due to seccomp filter change), and once all the TIF_SPEC_UPDATE tasks 
> >> schedule at least once, we're in a consistent state again and don't need 
> >> this, as every running task will then have its TIF consistent with MSR 
> >> value.
> > 
> > And how so? You set the bits is spec_flags. And then you set the TIF_UPDATE
> > bit which is evaluated once.
> > 
> > Then you OR the bits into tifp which is a local variable and has nothing to
> > do with the TIF flags of the next task. So on the next context switch this
> > will evaluate the previous state of the TIF bits and you could have spared
> > the whole exercise :)
> > 
> 
> This is better than my original implementation which was racy.
> Using task_spec_ssb_disable and task_spec_ib_disable to update TIF_* flags
> at context switch time makes the update logic very clear
> and extensible.

Clear yes. Extensible - hopefully not. This needs to end.

Thanks,

	tglx