linux-kernel - Re: [PATCH 1/7] atomic: Export fetch

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Date:	Tue, 24 Nov 2015 22:19:52 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/7] atomic: Export fetch_or()

On Tue, Nov 24, 2015 at 10:58:00AM -0500, Chris Metcalf wrote:
> On 11/13/2015 09:22 AM, Frederic Weisbecker wrote:
> >Export fetch_or() that's implemented and used internally by the
> >scheduler. We are going to use it for NO_HZ so make it generally
> >available.
> >
> >Cc: Christoph Lameter<cl@...ux.com>
> >Cc: Chris Metcalf<cmetcalf@...hip.com>
> >Cc: Ingo Molnar<mingo@...nel.org>
> >Cc: Luiz Capitulino<lcapitulino@...hat.com>
> >Cc: Peter Zijlstra<peterz@...radead.org>
> >Cc: Rik van Riel<riel@...hat.com>
> >Cc: Thomas Gleixner<tglx@...utronix.de>
> >Cc: Viresh Kumar<viresh.kumar@...aro.org>
> >Signed-off-by: Frederic Weisbecker<fweisbec@...il.com>
> >---
> >  include/linux/atomic.h | 18 ++++++++++++++++++
> >  kernel/sched/core.c    | 14 --------------
> >  2 files changed, 18 insertions(+), 14 deletions(-)
> >
> >diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> >index 00a5763..c3b99f8 100644
> >--- a/include/linux/atomic.h
> >+++ b/include/linux/atomic.h
> >@@ -451,6 +451,24 @@ static inline int atomic_dec_if_positive(atomic_t *v)
> >  }
> >  #endif
> >+/**
> >+ * fetch_or - perform *ptr |= mask and return old value of *ptr
> >+ * @ptr: pointer to value
> >+ * @mask: mask to OR on the value
> >+ *
> >+ * cmpxchg based fetch_or, macro so it works for different integer types
> >+ */
> >+#define fetch_or(ptr, mask)						\
> >+({	typeof(*(ptr)) __old, __val = *(ptr);				\
> >+	for (;;) {							\
> >+		__old = cmpxchg((ptr), __val, __val | (mask));		\
> >+		if (__old == __val)					\
> >+			break;						\
> >+		__val = __old;						\
> >+	}								\
> >+	__old;								\
> >+})
> >+
> >  #include <asm-generic/atomic-long.h>
> >  #ifdef CONFIG_GENERIC_ATOMIC64
> >  #include <asm-generic/atomic64.h>
> 
> I think this should be guarded by an "#ifndef" like other things in
> this file, so architectures can provide their own implementations,
> or can use the C11 atomic_fetch_or() for newer compilers.

Right.

> 
> Also, I wonder about the nomenclature here: other than cmpxchg
> and xchg, all the atomic ops are named "atomic_xxx".  For something
> that returns the old value, I'd expect it to be atomic_or_return()
> and be otherwise like the existing atomic_or() routine, and thus you'd
> specify "atomic_t tick_dependency".

I think Peterz needs it to be type-generic, like cmpxchg, such that he can
use it on tsk->thread_info->flags which type can vary. But if we happen to
need an atomic_t version, we can also provide an atomic_fetch_or() version.

Also note that or_return() means that you first do OR and then return the new value.

I remember debating a bit the name with Peterz and the current one now makes quite
some sense to me too :-)

Thanks!
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