Date: Mon, 23 Apr 2012 20:33:05 +0800 From: Fengwei yin <yfw.bsd@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-threads@freebsd.org, jack.ren@intel.com Subject: Re: About the memory barrier in BSD libc Message-ID: <CAPHpMumh3YpB3RDD-7g5tU6thiuNA6HTuVxmt-9_OzUiEdEXzA@mail.gmail.com> In-Reply-To: <20120423120720.GS2358@deviant.kiev.zoral.com.ua> References: <CAPHpMu=DOGQ=TuFeYH7bH8hVwteT4Q3k67-mvoOFob6P3Y506w@mail.gmail.com> <20120423084120.GD76983@zxy.spb.ru> <CAPHpMu=kCwhf1RV_sYBDWDPL8368YTMLXge4L_g_F4AkTX1H5g@mail.gmail.com> <20120423094043.GS32749@zxy.spb.ru> <CAPHpMukLUeetSKpH2oiKJQ3ML_PFHEi6a0hK3_Ery=LX1YEd3g@mail.gmail.com> <20120423113838.GT32749@zxy.spb.ru> <CAPHpMumWu_aaZ4Sj5Athro6441Y%2B3_phbD2jxkKE-CdBf-Fd8g@mail.gmail.com> <20120423120720.GS2358@deviant.kiev.zoral.com.ua>
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On Mon, Apr 23, 2012 at 8:07 PM, Konstantin Belousov
<kostikbel@gmail.com> wrote:
> On Mon, Apr 23, 2012 at 07:44:34PM +0800, Fengwei yin wrote:
>> On Mon, Apr 23, 2012 at 7:38 PM, Slawa Olhovchenkov <slw@zxy.spb.ru> wro=
te:
>> > On Mon, Apr 23, 2012 at 07:26:54PM +0800, Fengwei yin wrote:
>> >
>> >> On Mon, Apr 23, 2012 at 5:40 PM, Slawa Olhovchenkov <slw@zxy.spb.ru> =
wrote:
>> >> > On Mon, Apr 23, 2012 at 05:32:24PM +0800, Fengwei yin wrote:
>> >> >
>> >> >> On Mon, Apr 23, 2012 at 4:41 PM, Slawa Olhovchenkov <slw@zxy.spb.r=
u> wrote:
>> >> >> > On Mon, Apr 23, 2012 at 02:56:03PM +0800, Fengwei yin wrote:
>> >> >> >
>> >> >> >> Hi list,
>> >> >> >> If this is not correct question on the list, please let me know=
and
>> >> >> >> sorry for noise.
>> >> >> >>
>> >> >> >> I have a question regarding the BSD libc for SMP arch. I didn't=
see
>> >> >> >> memory barrier used in libc.
>> >> >> >> How can we make sure it's safe on SMP arch?
>> >> >> >
>> >> >> > /usr/include/machine/atomic.h:
>> >> >> >
>> >> >> > #define mb() =A0 =A0__asm __volatile("lock; addl $0,(%%esp)" : :=
: "memory")
>> >> >> > #define wmb() =A0 __asm __volatile("lock; addl $0,(%%esp)" : : :=
"memory")
>> >> >> > #define rmb() =A0 __asm __volatile("lock; addl $0,(%%esp)" : : :=
"memory")
>> >> >> >
>> >> >>
>> >> >> Thanks for the information. But it looks no body use it in libc.
>> >> >
>> >> > I think no body in libc need memory barrier: libc don't work with
>> >> > peripheral, for atomic opertions used different macros.
>> >>
>> >> If we check the usage of __sinit(), it is a typical singleton pattern=
which
>> >> needs memory barrier to make sure no potential SMP issue.
>> >>
>> >> Or did I miss something here?
>> >
>> > What architecture with cache incoherency and FreeBSD support?
>>
>> I suppose it's not related with cache inchoherency (I could be wrong).
>> It's related
>> with reorder of instruction by CPU.
>>
>> Here is the link talking about why need memory barrier for singleton:
>> http://www.oaklib.org/docs/oak/singleton.html
>>
>> x86 has strict memory model and may not suffer this kind of issue. But
>> ARM need to
>> take care of it IMHO.
>
> Please note that __sinit is idempotent, so double-initialization is not
> an issue there. The only possible problematic case would be other thread
> executing exit and not noticing non-NULL value for __cleanup while curren=
t
> thread just set it.
>
> I am not sure how much real this race is. Each call to _sinit() is immedi=
ately
> followed by a lock acquire, typically FLOCKFILE(), which enforces full ba=
rrier
> semantic due to pthread_mutex_lock call. The exit() performs __cxa_finali=
ze()
> call before checking __cleanup value, and __cxa_finalize() itself locks
> atexit_mutex. So the race is tiny and probably possible only for somewhat
> buggy applications which perform exit() while there are stdio operations
> in progress.
>
> Also note that some functions assign to __cleanup unconditionally.
>
> Do you see any real issue due to non-synchronized access to __cleanup ?
No. I didn't see real issue. I am just reviewing the code.
If you don't think __sinit has issue, let's check another code:
line 68 in libc/stdio/fclose.c
line 133 in libc/stdio/findfp.c (function __sfp())
Which is trying to free a fp slot by assign 0 to fp->_flags. But if
the instrucation
could be re-ordered, another CPU could see fp->_flags is assigned to 0
before the
cleanup from line 57 to 67.
Let's say, if another CPU is in line 133 of __sfp(), it could see
fp->_flags become
0 before it's aware of the cleanup (Line 57 to line 67 in
libc/stdio/fclose.c) happen.
Note: the mutex of FUNLOCKFILE(fp) in line 69 of libc/stdio/fclose.c
just could make sure
line 70 happen after line 68. It can't impact the re-order of line 57
~ line 68 by CPU.
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