Date: Wed, 25 Apr 2012 15:05:54 +0800 From: Fengwei yin <yfw.bsd@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: jack.ren@intel.com, freebsd-threads@freebsd.org Subject: Re: About the memory barrier in BSD libc Message-ID: <CAPHpMunhHr-yGR3vBZb176xFv_gugapm7w87P-LTciEbx%2BJHGg@mail.gmail.com> In-Reply-To: <20120425062627.GI2358@deviant.kiev.zoral.com.ua> References: <20120423084120.GD76983@zxy.spb.ru> <201204241343.q3ODhe2C032683@higson.cam.lispworks.com> <20120424140348.GY2358@deviant.kiev.zoral.com.ua> <201204241110.14017.jhb@freebsd.org> <20120424165842.GZ2358@deviant.kiev.zoral.com.ua> <201204241800.q3OI0X89007384@higson.cam.lispworks.com> <20120424181302.GB2358@deviant.kiev.zoral.com.ua> <CAPHpMu=Lv_zYqQEjbpSpJwYsmRr04714%2B_-2jhPqXdYj_LNgvQ@mail.gmail.com> <20120425062627.GI2358@deviant.kiev.zoral.com.ua>
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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.
> 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.
I wonder whether the rmb/wmb could be defined as a compiler barrier on
non-threaded case. Like STDIO_THREAD_LOCK which has version
for thread/non-thread case.
>>
>> diff --git a/lib/libc/stdio/fclose.c b/lib/libc/stdio/fclose.c
>> index f0629e8..a26f944 100644
>> --- a/lib/libc/stdio/fclose.c
>> +++ b/lib/libc/stdio/fclose.c
>> @@ -38,6 +38,7 @@ __FBSDID("$FreeBSD$");
>>
>> #include "namespace.h"
>> #include <errno.h>
>> +#include <machine/atomic.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include "un-namespace.h"
>> @@ -65,6 +66,7 @@ fclose(FILE *fp)
>> FREELB(fp);
>> fp->_file = -1;
>> fp->_r = fp->_w = 0; /* Mess up if reaccessed. */
>> + wmb();
>> fp->_flags = 0; /* Release this FILE for reuse. */
>> FUNLOCKFILE(fp);
>> return (r);
>> diff --git a/lib/libc/stdio/findfp.c b/lib/libc/stdio/findfp.c
>> index 89c0536..03b2945 100644
>> --- a/lib/libc/stdio/findfp.c
>> +++ b/lib/libc/stdio/findfp.c
>> @@ -129,9 +129,16 @@ __sfp()
>> */
>> THREAD_LOCK();
>> for (g = &__sglue; g != NULL; g = g->next) {
>> - for (fp = g->iobs, n = g->niobs; --n >= 0; fp++)
>> - if (fp->_flags == 0)
>> + for (fp = g->iobs, n = g->niobs; --n >= 0; fp++) {
>> + int __flags = fp->_flags;
>> + rmb();
>> + /*
>> + * If could see __flags is zero here, we are sure
>> + * the cleanup in fclose is done.
>> + */
>> + if (__flags == 0)
>> goto found;
>> + }
>> }
>> THREAD_UNLOCK(); /* don't hold lock while malloc()ing. */
>> if ((g = moreglue(NDYNAMIC)) == NULL)
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