linux-kernel - Re: [PATCH,RESEND] documentation: atomic_add

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Message-ID: <46CD2E11.6080301@yahoo.com.au>
Date:	Thu, 23 Aug 2007 16:49:53 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH,RESEND] documentation: atomic_add_unless() doesn't imply
 mb() on failure

Oleg Nesterov wrote:
> (the explicit ack/nack from maintainers is wanted).
> 
> A "typical" implementation of atomic_add_unless() can return 0 immediately
> after the first atomic_read() (before doing cmpxchg). In that case it doesn't
> provide any barrier semantics. See include/asm-ia64/atomic.h as an example.
> 
> We should either change the implementation, or fix the docs.

Did this end up getting merged? If not, it should, thanks.

Acked-by: Nick Piggin <npiggin@...e.de>

> 
> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> 
>  Documentation/atomic_ops.txt      |    3 ++-
>  Documentation/memory-barriers.txt |    2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff -puN Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/atomic_ops.txt
> --- a/Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure
> +++ a/Documentation/atomic_ops.txt
> @@ -137,7 +137,8 @@ If the atomic value v is not equal to u,
>  returns non zero. If v is equal to u then it returns zero. This is done as
>  an atomic operation.
>  
> -atomic_add_unless requires explicit memory barriers around the operation.
> +atomic_add_unless requires explicit memory barriers around the operation
> +unless it fails (returns 0).
>  
>  atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
>  
> diff -puN Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/memory-barriers.txt
> --- a/Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure
> +++ a/Documentation/memory-barriers.txt
> @@ -1492,7 +1492,7 @@ about the state (old or new) implies an 
>  	atomic_dec_and_test();
>  	atomic_sub_and_test();
>  	atomic_add_negative();
> -	atomic_add_unless();
> +	atomic_add_unless();	/* when succeeds (returns 1) */
>  	test_and_set_bit();
>  	test_and_clear_bit();
>  	test_and_change_bit();
> _
> 
> 


-- 
SUSE Labs, Novell Inc.
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