From: David Howells <dhowells@redhat.com>
To: Will Deacon <will@kernel.org>
Cc: dhowells@redhat.com, torvalds@linuxfoundation.org,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
peterz@infradead.org
Subject: Re: [RFC][PATCH] pipe: Convert ring to head/tail
Date: Tue, 17 Sep 2019 14:51:35 +0100 [thread overview]
Message-ID: <28447.1568728295@warthog.procyon.org.uk> (raw)
In-Reply-To: <20190915145905.hd5xkc7uzulqhtzr@willie-the-truck>
Will Deacon <will@kernel.org> wrote:
> > + /* Barrier: head belongs to the write side, so order reading
> > + * the data after reading the head pointer.
> > + */
> > + unsigned int head = READ_ONCE(pipe->head);
>
> Hmm, I don't understand this. Since READ_ONCE() doesn't imply a barrier,
> how are you enforcing the read-read ordering in the CPU?
It does imply a barrier: smp_read_barrier_depends(). I believe that's
> What is the purpose of saying "This may need to insert a barrier"? Can this
> function be overridden or something?
I mean it's arch-dependent whether READ_ONCE() inserts a barrier or not.
> Saying that "This inserts a barrier" feels misleading, because READ_ONCE()
> doesn't do that.
Yes it does - on the Alpha:
[arch/alpha/include/asm/barrier.h]
#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
[include/asm-generic/barrier.h]
#ifndef __smp_read_barrier_depends
#define __smp_read_barrier_depends() read_barrier_depends()
#endif
...
#ifndef smp_read_barrier_depends
#define smp_read_barrier_depends() __smp_read_barrier_depends()
#endif
[include/linux/compiler.h]
#define __READ_ONCE(x, check) \
({ \
union { typeof(x) __val; char __c[1]; } __u; \
if (check) \
__read_once_size(&(x), __u.__c, sizeof(x)); \
else \
__read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \
smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
__u.__val; \
})
#define READ_ONCE(x) __READ_ONCE(x, 1)
See:
commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author: Will Deacon <will.deacon@arm.com>
Date: Tue Oct 24 11:22:47 2017 +0100
locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()
David
next prev parent reply other threads:[~2019-09-17 13:52 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-13 13:00 [RFC][PATCH] pipe: Convert ring to head/tail David Howells
2019-09-13 13:06 ` My just-shovel-data-through-for-X-amount-of-time test David Howells
2019-09-15 14:59 ` [RFC][PATCH] pipe: Convert ring to head/tail Will Deacon
2019-09-17 13:51 ` David Howells [this message]
2019-09-17 17:07 ` Will Deacon
2019-09-18 15:43 ` Do we need to correct barriering in circular-buffers.rst? David Howells
2019-09-18 16:48 ` Linus Torvalds
2019-09-19 13:59 ` David Howells
2019-09-19 15:59 ` Linus Torvalds
2019-09-23 14:49 ` Peter Zijlstra
2019-09-27 9:51 ` Andrea Parri
2019-09-27 12:49 ` Peter Zijlstra
2019-09-27 15:57 ` Peter Zijlstra
2019-09-27 20:43 ` Nick Desaulniers
2019-09-27 21:58 ` Nick Desaulniers
2019-09-30 9:33 ` Peter Zijlstra
2019-09-30 11:54 ` Peter Zijlstra
2019-09-30 12:02 ` Peter Zijlstra
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