Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

[Will Deacon]

On Wed, Feb 20, 2019 at 02:55:59PM -0800, H. Peter Anvin wrote:
> On 2/19/19 4:48 AM, Will Deacon wrote:
> > 
> > I think you'll still hate this, but could we not disable preemption during
> > the uaccess-enabled region, re-enabling it on the fault path after we've
> > toggled uaccess off and disable it again when we return back to the
> > uaccess-enabled region? Doesn't help with tracing, but it should at least
> > handle the common case.
> > 
> 
> There is a worse problem with this, I still realize: this would mean blocking
> preemption across what could possibly be a *very* large copy_from_user(), for
> example.

I don't think it's legitimate to call copy_{to,from}_user() inside a
user_access_{begin,end} region. You'd need to add some unsafe variants,
which could periodically disable uaccess and call cond_resched() inside
the loop to avoid the problem you're eluding to.

For existing callers of copy_{to,from}_user(), there's no issue as they
don't call into the scheduler during the copy operation. Exceptions are
handled fine by the code in mainline today.

GCC plugins are a cool idea, but I'm just a bit nervous about relying on
them for things like this.

Will