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

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Message-ID: <20190221124629.GA12696@fuggles.cambridge.arm.com>
Date:   Thu, 21 Feb 2019 12:46:29 +0000
From:   Will Deacon <will.deacon@....com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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