linux-kernel - Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

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Message-ID: <20190228152432.GC32494@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Feb 2019 16:24:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     bp@...en8.de, brgerst@...il.com, catalin.marinas@....com,
        dvlasenk@...hat.com, hpa@...or.com, james.morse@....com,
        jpoimboe@...hat.com, julien.thierry@....com, luto@...capital.net,
        luto@...nel.org, mingo@...nel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, valentin.schneider@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

On Thu, Feb 28, 2019 at 03:10:44PM +0000, Chris Wilson wrote:
> Quoting Peter Zijlstra (2019-02-28 14:54:56)
> > drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x3c: redundant UACCESS disable
> > drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x66: redundant UACCESS disable
> > 
> > AKA. you don't need user_access_end() if user_access_begin() fails.
> 
> Cool, I had no idea if it was required or not. The closest to
> instructions on how to use user_access_begin() is in
> arch/x86/include/asm/uaccess.h ... which I guess is earlier in this
> series!

Ah, I don't think I put the rules in a comment; that would've been
useful I suppose ;-)

I did teach the rules to objtool though, and that is in the next patch:

  https://lkml.kernel.org/r/20190228150152.635362492@infradead.org

so at least it will yell at you during compile time when you get it
wrong.

> > Cc: Chris Wilson <chris@...is-wilson.co.uk>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> Reviewed-by: Chris Wilson <chris@...is-wilson.co.uk>

Thanks!