linux-kernel - RE: [PATCH 3/7] microblaze: Define SMP safe bit operations

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Date:   Thu, 13 Feb 2020 09:01:21 +0000
From:   Stefan Asserhall <stefana@...inx.com>
To:     Michal Simek <michals@...inx.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Simek <michals@...inx.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "monstr@...str.eu" <monstr@...str.eu>, git <git@...inx.com>,
        "arnd@...db.de" <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Will Deacon <will@...nel.org>
Subject: RE: [PATCH 3/7] microblaze: Define SMP safe bit operations

> On 12. 02. 20 16:53, Peter Zijlstra wrote:
> > On Wed, Feb 12, 2020 at 04:42:25PM +0100, Michal Simek wrote:
> >> From: Stefan Asserhall <stefan.asserhall@...inx.com>
> >>
> >> For SMP based system there is a need to have proper bit operations.
> >> Microblaze is using exclusive load and store instructions.
> >>
> >> Signed-off-by: Stefan Asserhall <stefan.asserhall@...inx.com>
> >> Signed-off-by: Michal Simek <michal.simek@...inx.com>
> >
> >> +/*
> >> + * clear_bit doesn't imply a memory barrier  */
> >> +#define smp_mb__before_clear_bit()	smp_mb()
> >> +#define smp_mb__after_clear_bit()	smp_mb()
> >
> > These macros no longer exist.
> 
> ok. Easy to remove.
> 
> >
> > Also, might I draw your attention to:
> >
> >   include/asm-generic/bitops/atomic.h
> >
> > This being a ll/sc arch, I'm thinking that if you do your atomic_t
> > implementation right, the generic atomic bitop code should be near
> > optimal.
> >
> 
> Based on my look it looks like that I can replace implementations in this file by
> sourcing which will be using atomic operations.
> 
> #include <asm-generic/bitops/atomic.h>
> #include <asm-generic/bitops/lock.h>
> 
> Correct?
> 
> Would be good to run any testsuite to prove that all operations works as
> expected. Is there any testsuite I can use to confirm it?
> 
> Thanks,
> Michal

The comment in the generic bitops.h says "You should recode these in the
native assembly language, if at all possible". I don't think using the generic
implementation will be as efficient as the current arch specific one.

My recommendation is to stick with the arch specific implementation.

Thanks,
Stefan