Date: Mon, 26 Nov 2007 00:31:22 -0500 (EST) From: Daniel Eischen <eischen@vigrid.com> To: Julian Elischer <julian@elischer.org> Cc: Stephan Uphoff <ups@freebsd.org>, cvs-src@freebsd.org, src-committers@freebsd.org, cvs-all@freebsd.org Subject: Re: cvs commit: src/share/man/man9 locking.9 rmlock.9 src/sys/conf files src/sys/kern kern_rmlock.c subr_lock.c subr_pcpu.c subr_smp.c src/sys/sys _rmlock.h lock.h pcpu.h rmlock.h smp.h Message-ID: <Pine.GSO.4.64.0711260008090.11036@sea.ntplx.net> In-Reply-To: <4749B971.3000703@elischer.org> References: <200711081447.lA8EltXO052057@repoman.freebsd.org> <47492064.7080108@freebsd.org> <Pine.GSO.4.64.0711251207590.8538@sea.ntplx.net> <4749B971.3000703@elischer.org>
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On Sun, 25 Nov 2007, Julian Elischer wrote:
> Daniel Eischen wrote:
>> On Sat, 24 Nov 2007, Darren Reed wrote:
>>
>>> Stephan Uphoff wrote:
>>>> ups 2007-11-08 14:47:55 UTC
>>>>
>>>> FreeBSD src repository
>>>>
>>>> Modified files:
>>>> share/man/man9 locking.9 sys/conf files
>>>> sys/kern subr_lock.c subr_pcpu.c subr_smp.c sys/sys
>>>> lock.h pcpu.h smp.h Added files:
>>>> share/man/man9 rmlock.9 sys/kern kern_rmlock.c
>>>> sys/sys _rmlock.h rmlock.h Log:
>>>> Initial checkin for rmlock (read mostly lock) a multi reader single
>>>> writer
>>>> lock optimized for almost exclusive reader access. (see also rmlock.9)
>>>>
>>>
>>> Is there a white paper or other documentation around somewhere that
>>> discusses the benefits/tradeoffs with using rmlock vs rwlock?
>>
>> Why aren't we using the rwlock interfaces, but just allowing a different
>> behavior when the lock is created (rwlock_init2() or something)? It
>> would seem simpler to keep the same interface and allow easy toggling
>> between rwlocks and rmlocks. The same way we can initialize kernel
>> mutexes differently (MTX_DEF, MTX_SPIN) could be applied here.
>>
>
> I think that If anything, we should be going in the other direction..
> firstly, mutexes are just rw_locks with no readers. So we might
> as well make them the same thing..
Robert already answered my question sufficiently...
I am looking at the way Solaris does its kernel synchronizations.
They have mutexes, CVs, and rwlocks as far as I can see. It is very
convenient to have mutexes and CVs just as their userland counterparts.
Mutexes are mutual exclusion and by definition not rwlock-capable.
You don't want to mix the mutex API with the rwlock or rmlock APIs,
and also, the CV APIs (cv_waitXXX, cv_timedwaitXXX) only take mutex
types as arguments. If the implementations can share code, ok, but
don't share the mutex/CV APIs with the r{mw}lock APIs :-)
> Spin and blocking mutexes should in my opinion be defined as different
> structures, at least in name so that the compiler hits you with a clue-bat
> when you try use a spin-lock with non-spinlock ops etc.
That seems nice, but doesn't go along well with trying to keep a
similar API as Solaris. It is convenient to share the APIs so
that it is easy to change the mtx_init() from a default to a spin
type without changing the rest of the code. We really shouldn't
have a need for spin mutexes if the default mutexes are adaptive,
and if mutexes taken by interrupt handlers are initialized in a
manner similar to Solaris. There really shouldn't be a separate
API for spin mutexes. Once a mutex is initialized as MTX_DEF
or MTX_SPIN, that should be sufficient.
> not sure why sx-locks exist at all, as they seem to be a variant of sleep.
> I think it's just a convenience function set to allow one to implement
> a sleep-derived synchronisation.
Hmm, sx locks seem similar to rmlocks, at least according to the
description:
Shared/exclusive locks are used to protect data that are read far
more often than they are written.
Do we need both?
--
DE
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