Date: Sat, 24 Nov 2007 16:25:22 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Attilio Rao <attilio@freebsd.org> Cc: Stephan Uphoff <ups@freebsd.org>, Max Laier <max@love2party.net>, Alfred Perlstein <alfred@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: rwlocks, correctness over speed. Message-ID: <20071124162322.V14018@fledge.watson.org> In-Reply-To: <3bbf2fe10711240553k1eb35a5au23cae8af08f5864c@mail.gmail.com> References: <20071121222319.GX44563@elvis.mu.org> <3bbf2fe10711220753u435ff4cbxa94d5b682292b970@mail.gmail.com> <200711221726.27108.max@love2party.net> <20071123082339.GN44563@elvis.mu.org> <47469328.8020404@freebsd.org> <20071123092415.GP44563@elvis.mu.org> <4746F858.4070301@freebsd.org> <20071123235346.E14018@fledge.watson.org> <3bbf2fe10711231930m459dc800wbbb894b9fd50ca13@mail.gmail.com> <20071124103231.A14018@fledge.watson.org> <3bbf2fe10711240553k1eb35a5au23cae8af08f5864c@mail.gmail.com>
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On Sat, 24 Nov 2007, Attilio Rao wrote:
>> I must have missed recursion arriving then -- I'll modify uipc_usrreq.c to
>> set the recursion flag on the rwlock in UNIX domain sockets rather than
>> doing the nasty hack that was previously required. At the time, the hack
>> was added because it seemed recursion was not going to be added to rwlocks,
>> but sonewconn() behavior for listen sockets really ended up requiring it.
>
> attilio 2007-06-26 21:31:56 UTC
>
> FreeBSD src repository
>
> Modified files:
> sys/kern kern_rwlock.c
> sys/sys _rwlock.h rwlock.h
> Log:
> Introduce a new rwlocks initialization function: rw_init_flags.
> This is very similar to sx_init_flags: it initializes the rwlock using
> special flags passed as third argument (RW_DUPOK, RW_NOPROFILE,
> RW_NOWITNESS, RW_QUIET, RW_RECURSE).
> Among these, the most important new feature is probabilly that rwlocks
> can be acquired recursively now (for both shared and exclusive paths).
Yes, that was four months after I added rw_wowned(9) to work around the lack
of recursion support. :-) However, it looks like the man page was never
updated? It contains the following rather explicit language:
Another important property is that shared holders of rwlock can recurse,
but exclusive locks are not allowed to recurse.
Robert N M Watson
Computer Laboratory
University of Cambridge
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