From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kent Overstreet Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting Date: Thu, 29 Nov 2012 10:57:20 -0800 Message-ID: <20121129185720.GE15094@google.com> References: <1354121029-1376-1-git-send-email-koverstreet@google.com> <1354121029-1376-23-git-send-email-koverstreet@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, zab@redhat.com, bcrl@kvack.org, jmoyer@redhat.com, axboe@kernel.dk, viro@zeniv.linux.org.uk To: Andi Kleen Return-path: Content-Disposition: inline In-Reply-To: Sender: owner-linux-aio@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Nov 29, 2012 at 10:45:04AM -0800, Andi Kleen wrote: > Kent Overstreet writes: > > > This implements a refcount with similar semantics to > > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t > > but dynamically switches to per cpu refcounting when the rate of > > gets/puts becomes too high. > > This will only work if you put on the same CPU as you get, right? Nope, no such restriction. > In this case I would rather use RCU. It's clearly unusable for anything > blocking (or not get_cpu) Normally RCU already handles the "ref count for short non > blocking case" The kioctx refcount isn't held for short nonblocking duration, io_getevents() holds it and may block for arbitrarily long. Maybe SRCU could be made to work for it (I havent' really looked at the RCU variants) but it doesn't seem like a good idea. This thing really is just a refcount, the percpu part isn't exposed to the user at all. -- To unsubscribe, send a message with 'unsubscribe linux-aio' in the body to majordomo@kvack.org. For more info on Linux AIO, see: http://www.kvack.org/aio/ Don't email: aart@kvack.org