From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Date: Fri, 19 Dec 2008 03:35:14 +0000 Subject: Re: local_add_return Message-Id: <20081219033514.GA7162@Krystal> List-Id: References: <200812170908.05423.rusty@rustcorp.com.au> <20081217000155.GA28174@Krystal> <200812190922.57629.rusty@rustcorp.com.au> In-Reply-To: <200812190922.57629.rusty@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Rusty Russell Cc: David Miller , rostedt@goodmis.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, paulus@samba.org, benh@kernel.crashing.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, Christoph Lameter , "Paul E. McKenney" , Martin Bligh * Rusty Russell (rusty@rustcorp.com.au) wrote: > On Wednesday 17 December 2008 10:31:55 Mathieu Desnoyers wrote: > > I think we have two different use-cases here : > > > > - local_t is useful as-is for things such as a tracer, which need to > > modify an element of data atomically wrt local interrupts. The > > atomic_long_t, in this case, is the correct fallback. > > - local_count_t could be used for fast counters. > > Hi Mathieu, > > Complete agreement. > > I guess I'm biassed towards local_t = counter version, something else > = nmi-safe version because that's what it was originally. Looking through > the tree, there are only 5 users: module, dmaengine and percpu_counter want > a counter, and tracing and x86 nmi.c want nmi-safe. There are several other > places I know of which want local_t-the-counter. > > I'll prepare a patch which adds nmi_safe_t, and see how it looks. There's > no amazing hurry on this, so I won't race to hit the merge window. > OK, But can we turn what you call "nmi_safe_t" into "local_atomic_t" then ? Because we have to specify that this type must only be used as part of per-cpu data with preemption disabled, and we also specify that it is atomic. Plus, nmi_safe_t does not make much sense on architectures without NMIs, where we sometimes disable interrupts to make the modification "atomic" wrt all other interrupts that can happen. Mathieu > Thanks! > Rusty. -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68