* ops_sem and tpm_mutex @ 2016-07-05 11:06 Jarkko Sakkinen [not found] ` <20160705110647.GA28275-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> 0 siblings, 1 reply; 2+ messages in thread From: Jarkko Sakkinen @ 2016-07-05 11:06 UTC (permalink / raw) To: tpmdd-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f Hi I started to looking at ops_sem and tpm_mutex because it would be nicer to have one lock. When we do something that uses ops_sem we do this: 1. read lock for ops_sem 2. lock tpm_mutex This is the basic pattern. Basically we always loose the benefit of RW-lock because in every use case we also lock a mutex. And the mutex of course cannot be taken off because we want to mutually exclude the TPM access. 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(). /Jarkko ------------------------------------------------------------------------------ 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 ^ permalink raw reply [flat|nested] 2+ messages in thread
[parent not found: <20160705110647.GA28275-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]
* Re: ops_sem and tpm_mutex [not found] ` <20160705110647.GA28275-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> @ 2016-07-05 16:01 ` Jason Gunthorpe 0 siblings, 0 replies; 2+ messages in thread From: Jason Gunthorpe @ 2016-07-05 16:01 UTC (permalink / raw) To: Jarkko Sakkinen; +Cc: tpmdd-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f 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 ^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2016-07-05 16:01 UTC | newest] Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2016-07-05 11:06 ops_sem and tpm_mutex Jarkko Sakkinen [not found] ` <20160705110647.GA28275-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> 2016-07-05 16:01 ` Jason Gunthorpe
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