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Date: Wed, 30 Apr 2014 09:17:37 -0700 From: Davidlohr Bueso <davidlohr@...com> To: Peter Zijlstra <peterz@...radead.org> Cc: Ingo Molnar <mingo@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Tim Chen <tim.c.chen@...ux.intel.com>, Andrea Arcangeli <aarcange@...hat.com>, Alex Shi <alex.shi@...aro.org>, Andi Kleen <andi@...stfloor.org>, Michel Lespinasse <walken@...gle.com>, Rik van Riel <riel@...hat.com>, Peter Hurley <peter@...leysoftware.com>, Thomas Gleixner <tglx@...utronix.de>, "Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>, Aswin Chandramouleeswaran <aswin@...com>, "Norton, Scott J" <scott.norton@...com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2] rwsem: Support optimistic spinning On Wed, 2014-04-30 at 10:27 +0200, Peter Zijlstra wrote: > On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote: > > +/* > > + * Try to acquire write lock before the writer has been put on wait queue. > > + */ > > +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > > +{ > > + long count = ACCESS_ONCE(sem->count); > > +retry: > > + if (count == RWSEM_WAITING_BIAS) { > > + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS, > > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS); > > + /* allow write lock stealing, try acquiring the write lock. */ > > + if (count == RWSEM_WAITING_BIAS) > > + goto acquired; > > + else if (count == 0) > > + goto retry; > > + } else if (count == 0) { > > + count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS); > > + if (count == 0) > > + goto acquired; > > + else if (count == RWSEM_WAITING_BIAS) > > + goto retry; > > + } > > + return false; > > + > > +acquired: > > + return true; > > +} > > Could we have written that like: > > static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > { > long old, count = ACCESS_ONCE(sem->count); > > for (;;) { > if (!(count == 0 || count == RWSEM_WAITING_BIAS)) > return false; > > old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS); > if (old == count) > return true; > > count = old; > } > } > > ? Yeah that's a lot nicer. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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