Date: Wed, 25 Apr 2012 10:12:36 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-threads@freebsd.org Cc: jack.ren@intel.com Subject: Re: About the memory barrier in BSD libc Message-ID: <201204251012.36754.jhb@freebsd.org> In-Reply-To: <CAPHpMunhHr-yGR3vBZb176xFv_gugapm7w87P-LTciEbx%2BJHGg@mail.gmail.com> References: <20120423084120.GD76983@zxy.spb.ru> <20120425062627.GI2358@deviant.kiev.zoral.com.ua> <CAPHpMunhHr-yGR3vBZb176xFv_gugapm7w87P-LTciEbx%2BJHGg@mail.gmail.com>
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On Wednesday, April 25, 2012 3:05:54 am Fengwei yin wrote: > On Wed, Apr 25, 2012 at 2:26 PM, Konstantin Belousov > <kostikbel@gmail.com> wrote: > > On Wed, Apr 25, 2012 at 11:25:40AM +0800, Fengwei yin wrote: > >> On Wed, Apr 25, 2012 at 2:13 AM, Konstantin Belousov > >> <kostikbel@gmail.com> wrote: > >> > On Tue, Apr 24, 2012 at 07:00:33PM +0100, Martin Simmons wrote: > >> >> >>>>> On Tue, 24 Apr 2012 19:58:42 +0300, Konstantin Belousov said: > >> >> > > >> >> > + > >> >> > + /* > >> >> > + * Lock the spinlock used to protect __sglue list walk in > >> >> > + * __sfp(). The __sfp() uses fp->_flags == 0 test as an > >> >> > + * indication of the unused FILE. > >> >> > + * > >> >> > + * Taking the lock prevents possible compiler or processor > >> >> > + * reordering of the writes performed before the final _flags > >> >> > + * cleanup, making sure that we are done with the FILE before > >> >> > + * it is considered available. > >> >> > + */ > >> >> > + STDIO_THREAD_LOCK(); > >> >> > fp->_flags = 0; /* Release this FILE for reuse. */ > >> >> > + STDIO_THREAD_UNLOCK(); > >> >> > FUNLOCKFILE(fp); > >> >> > return (r); > >> >> > >> >> Is that assumption about reordering correct? I.e. is FreeBSD's spinlock a > >> >> two-way barrier? > >> >> > >> >> It isn't unusual for the locking primitive to be a one-way barrier, i.e. (from > >> >> Linux kernel memory-barriers.txt) "Memory operations that occur before a LOCK > >> >> operation may appear to happen after it completes." See also acq and rel in > >> >> atomic(9). > >> > Taking the lock prevents the __sfp from iterating the list until the > >> > spinlock is released. Since release makes sure that all previous memory > >> > operations become visible, the acquire in the spinlock lock provides > >> > the neccesary guarentee. > >> > >> IMHO, the lock to me is too heavy here. What about this patch? > >> > >> NOTE: patch just show thoughts. I didn't even check build checking. > > Yes, it might be correct. But FreeBSD does prefer the acq/rel barriers > > over the rmb/wmb. > > > > There is no stand alone acq/rel APIs (Sorry, as new guy to FreeBSD, > don't know too much APIs). They are bound to atomic operations. > And yes, atomic_acq/rel should work also. Yes, you would want to use atomic_store_rel() or some such. Often doing so is much clearer than stand-alone membar's as it indicates which write has special ordering. > > Also, the lock is not that heavy right there, and the committed patch > > provides mostly zero overhead for non-threaded case. > > But lock could introduced contention in SMP case which could be avoid > with rmb/wmb. Seriously, if you are using stdio, you've already given up on performance enough to not care about contention for fclose() vs fopen(). -- John Baldwin
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