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Date: Mon, 5 Apr 2010 18:08:22 -0400 From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Josh Triplett <josh@...htriplett.org> Cc: Randy Dunlap <randy.dunlap@...cle.com>, mingo@...e.hu, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>, Steven Rostedt <rostedt@...dmis.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Nicholas Miell <nmiell@...cast.net>, Linus Torvalds <torvalds@...ux-foundation.org>, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com, Nick Piggin <npiggin@...e.de>, Chris Friesen <cfriesen@...tel.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] introduce sys_membarrier(): process-wide memory barrier (v10) * Josh Triplett (josh@...htriplett.org) wrote: > On Mon, Apr 05, 2010 at 03:10:57PM -0400, Mathieu Desnoyers wrote: > > * Randy Dunlap (randy.dunlap@...cle.com) wrote: > > > On Mon, 5 Apr 2010 13:57:37 -0400 Mathieu Desnoyers wrote: > > > > +/* > > > > + * sys_membarrier - issue memory barrier on current process running threads > > > > + * @flags: One of these must be set: > > > > + * MEMBARRIER_EXPEDITED > > > > + * Adds some overhead, fast execution (few microseconds) > > > > + * MEMBARRIER_DELAYED > > > > + * Low overhead, but slow execution (few milliseconds) > > > > + * > > > > + * MEMBARRIER_QUERY > > > > + * This optional flag can be set to query if the kernel supports > > > > + * a set of flags. > > > > + * > > > > + * return values: Returns -EINVAL if the flags are incorrect. Testing for kernel > > > > + * sys_membarrier support can be done by checking for -ENOSYS return value. > > > > + * Return values >= 0 indicate success. For a given set of flags on a given > > > > + * kernel, this system call will always return the same value. It is therefore > > > > + * correct to check the return value only once at library load, passing the > > > > > > library load assumes caller is a library? does the kernel care about that? > > > > Nope, it doesn't. Will rephrase: > > > > ... It is therefore > > * correct to check the return value only once during a process lifetime, > > * passing the MEMBARRIER_QUERY flag in addition to only check if the flags are > > * supported, without performing any synchronization. > > Technically you can optimize even more than "process lifetime", since as > you said the results hold "For a given set of flags on a given kernel". > So you could check once and use the results as long as you remain on the > same running system. (Craziness like live process migration and > checkpoint/restart aside. :) ) True. But notice that I first state the general correctness condition (consistent on a given kernel), and then state that it is therefore true over a process life-time. Basically, I want to clarify how the "MEMBARRIER_QUERY" flag can be used from a user-space perspective. Stating that we can save the result here and there and re-use it afterward will not help making this system call documentation clearer; I fear it would just have the opposite effect: confusing API users. Thanks, Mathieu > > - Josh Triplett -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- 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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