Date: Thu, 21 Dec 2006 14:58:02 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Suleiman Souhlal" <ssouhlal@freebsd.org> Cc: freebsd-hackers@freebsd.org, Duane Whitty <duane@dwlabs.ca> Subject: Re: Locking fundamentals Message-ID: <3bbf2fe10612210558m66795673kd352a385a98f6e2b@mail.gmail.com> In-Reply-To: <458A249D.3030502@FreeBSD.org> References: <20061220041843.GA10511@dwpc.dwlabs.ca> <3bbf2fe10612200414j4c1c01ecr7b37e956b70b01fa@mail.gmail.com> <458A249D.3030502@FreeBSD.org>
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2006/12/21, Suleiman Souhlal <ssouhlal@freebsd.org>: > Attilio Rao wrote: > > 2006/12/20, Duane Whitty <duane@dwlabs.ca>: > > > >> Hello again, > >> > >> It seems to me that understanding locking holds the key to > >> understanding fbsd internals. > >> > >> Could someone review my understanding of fbsd locking fundamentals. > >> (No assertions here, just questions) > >> > >> lock_mgr > >> -------------------- > >> mutexes|sx_lock > >> ------------------- ^ > >> atomic | mem barriers | > > > > > > Our current locking hierarchy is basically different: > > > > III level: lockmgr - sema - sx > > II level: mutex (sleep/spin/pool) - rwlock - refcount - cv - msleep > > I level: atomic instructions - memory barriers - sleepqueues/turnstiles > > > > (a lower lever means that the upper layer primitives use it as a base. > > ie: sx locks are build using 1 pool > > mutex and 2 condition variables). > > > > This scheme is far from being perfect due to the presence of 'level 3 > > primitives' which should never exist. > > Currently, there is an ongoing efforts to take all the top layer > > primitives to the level II. > > > > On the other side, level I primitives should never be used directly by > > kernel code, but should only be used as a bottom layer for > > syncronizing primitives. All you need to care is in the layer 2 and 3 > > (and possibly should switch to layer 2). > > I disagree. There are many uses of atomic operations in the kernel that are not for locks or refcounts. It's a bad idea to use locks if you can achieve the same thing locklessly, with atomic operations. I can agree with you about this but atomic instructions/memory barriers should be used very carefully (and if you know what you are going to do). It is very simple to write down a wrong semantic using them. > I would personally also add "critical sections" (critical_enter()/critical_exit()) at level I. They can be used instead of locks when you know your data will only be accessed on one CPU, and you only need to protect it from (non-FAST) interrupt handlers. >From this point of view, we would also add sched_pin()/sched_unpin() which are used in order to avoid thread migration between CPUs (particulary helpfull in the case we have to access safely to some per-CPU datas). However, probabilly one of the most important usage we do of critical_section is in the spin mutex implementation (which linked to interrupt disabling would avoid deadlock in spin mutex code). Attilio -- Peace can only be achieved by understanding - A. Einstein
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