linux-kernel - Re: [RFC PATCH v2 13/13] powerpc/kernel: Do not use READ_ONCE() to access current thread

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Message-Id: <1586137083.v4cw2k5x9e.astroid@bobo.none>
Date:   Mon, 06 Apr 2020 11:39:19 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>, msuchanek@...e.de,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v2 13/13] powerpc/kernel: Do not use READ_ONCE() to
 access current thread_info flags

Christophe Leroy's on April 6, 2020 3:44 am:
> current is a volatile pointer hold by r2 register.
> 
> READ_ONCE() is not required.
> 
> Before: 327 cycles on null_syscall
> Before: 325 cycles on null_syscall

Patches against Michael's next-test branch have a few small changes 
here. I need to look at the fallout on 64-bit code, although most
likely it's not measurable so if it saves you a few cycles we can
lean toward what ppc32 prefers.

Same with the compat test.

Thanks,
Nick