From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: ops_sem and tpm_mutex Date: Tue, 5 Jul 2016 10:01:00 -0600 Message-ID: <20160705160100.GA715@obsidianresearch.com> References: <20160705110647.GA28275@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20160705110647.GA28275-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: tpmdd-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org To: Jarkko Sakkinen Cc: tpmdd-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: tpmdd-devel@lists.sourceforge.net On Tue, Jul 05, 2016 at 02:06:47PM +0300, Jarkko Sakkinen wrote: > This is the basic pattern. Basically we always loose the benefit of > RW-lock because in every use case we also lock a mutex. The only purpose of the rw lock is protect against unregister, and that is the benifit it provides. > What I was thinking that maybe we could have kref for ops instead > of lock. In the places where we now use read lock you could use > kref_get_unless_zero() to avoid races with tpm_chip_unregister(). No, it needs to be a lock, the unregister path must block and sleep, and a kref cannot do that alone, by the time you build in the locking you've made something more expensive than a rwlock. The performance alternative is to use srcu for the rwlock, but since we don't really have a performance concern in TPM I would rather not see such complexity. Another alternative would be to merge the rw-lock and mutex together (ie hold mutex before touching ops at all), however this semantically changes things by linking the lifetime and concurrancy models together. I belive I looked at that before doing the rwsem and decided it was a huge amount of tricky work. Jason ------------------------------------------------------------------------------ Attend Shape: An AT&T Tech Expo July 15-16. Meet us at AT&T Park in San Francisco, CA to explore cutting-edge tech and listen to tech luminaries present their vision of the future. This family event has something for everyone, including kids. Get more information and register today. http://sdm.link/attshape