Date: Tue, 3 Jan 2017 20:59:51 +0000 (UTC) From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r311168 - head/share/man/man9 Message-ID: <201701032059.v03Kxpgs093283@repo.freebsd.org>
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Author: mjg Date: Tue Jan 3 20:59:50 2017 New Revision: 311168 URL: https://svnweb.freebsd.org/changeset/base/311168 Log: Add the upcoming atomic_fcmpset family to the atomic(9) man page. These primitives give the caller the read value if the exchange attempt failed which saves an explicit reload for cmpset loops. The man page was partially submitted by kib. Reviewed by: kib (previous version), jhb (previous version) Modified: head/share/man/man9/atomic.9 Modified: head/share/man/man9/atomic.9 ============================================================================== --- head/share/man/man9/atomic.9 Tue Jan 3 20:28:48 2017 (r311167) +++ head/share/man/man9/atomic.9 Tue Jan 3 20:59:50 2017 (r311168) @@ -23,13 +23,14 @@ .\" .\" $FreeBSD$ .\" -.Dd May 12, 2016 +.Dd Jan 3, 2017 .Dt ATOMIC 9 .Os .Sh NAME .Nm atomic_add , .Nm atomic_clear , .Nm atomic_cmpset , +.Nm atomic_fcmpset , .Nm atomic_fetchadd , .Nm atomic_load , .Nm atomic_readandclear , @@ -50,6 +51,12 @@ .Fa "<type> old" .Fa "<type> new" .Fc +.Ft int +.Fo atomic_fcmpset_[acq_|rel_]<type> +.Fa "volatile <type> *dst" +.Fa "<type> *old" +.Fa "<type> new" +.Fc .Ft <type> .Fn atomic_fetchadd_<type> "volatile <type> *p" "<type> v" .Ft <type> @@ -228,6 +235,45 @@ functions are not implemented for the ty and .Dq Li 16 . .Bl -hang +.It Fn atomic_fcmpset dst *old new +.El +.Pp +On architectures implementing +.Em Compare And Swap +operation in hardware, the functionality can be described as +.Bd -literal -offset indent -compact +if (*dst == *old) { + *dst = new; + return (1); +} else { + *old = *dst; + return (0); +} +.Ed +On architectures which provide +.Em Load Linked/Store Conditional +primitive, the write to +.Dv *dst +might also fail for several reasons, most important of which +is a parallel write to +.Dv *dst +cache line by other CPU. +In this case +.Fn atomic_fcmpset +function also returns +.Dv false , +despite +.Dl *old == *dst . +.Pp +The +.Fn atomic_fcmpset +functions are not implemented for the types +.Dq Li char , +.Dq Li short , +.Dq Li 8 , +and +.Dq Li 16 . +.Bl -hang .It Fn atomic_fetchadd p v .Bd -literal -compact tmp = *p; @@ -353,6 +399,16 @@ The .Fn atomic_cmpset function returns the result of the compare operation. The +.Fn atomic_fcmpset +function returns +.Dv true +if the operationg succeeded. +Otherwise it returns +.Dv false +and sets +.Va *old +to the found value. +The .Fn atomic_fetchadd , .Fn atomic_load , .Fn atomic_readandclear ,
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