Date: Tue, 17 Nov 2009 20:53:50 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: jilles@stack.nl Cc: svn-src-head@FreeBSD.org, lynx.ripe@gmail.com, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, edwin@FreeBSD.org Subject: Re: svn commit: r194783 - head/lib/libc/stdtime Message-ID: <20091117.205350.1682637096.imp@bsdimp.com> In-Reply-To: <20091117182501.GA70742@stack.nl> References: <4B01E548.7040708@gmail.com> <20091117182501.GA70742@stack.nl>
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In message: <20091117182501.GA70742@stack.nl>
Jilles Tjoelker <jilles@stack.nl> writes:
: On Tue, Nov 17, 2009 at 01:50:32AM +0200, Dmitry Pryanishnikov wrote:
: > > Author: edwin
: > > Date: Tue Jun 23 22:28:44 2009
: > > New Revision: 194783
: > > URL: http://svn.freebsd.org/changeset/base/194783
:
: > > Log:
: > > Remove duplicate if-statement on gmt_is_set in gmtsub().
:
: > > MFC after: 1 week
:
: > > Modified:
: > > head/lib/libc/stdtime/localtime.c
:
: > This change looks like a (small?) pessimization to me: before it,
: > _MUTEX_LOCK/_MUTEX_UNLOCK pair would be skipped for the case gmt_is_set
: > == TRUE (all invocations except the first one), now it won't. I'm not
: > sure whether this is critical here though...
:
: It is certainly less efficient, but the old code was (most likely)
: wrong. It used an idiom known as "double checked locking", which is
: incorrect in most memory models. The problem is that the store to
: gmt_is_set may become visible without stores to other memory (gmtptr and
: what it points to) becoming visible.
Wouldn't a memory barrier solve that at a much lower cost than a full
lock?
Warner
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