Date: Tue, 24 Apr 2012 14:43:40 +0100 From: Martin Simmons <martin@lispworks.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: <201204241343.q3ODhe2C032683@higson.cam.lispworks.com> In-Reply-To: <20120423130343.GT2358@deviant.kiev.zoral.com.ua> (message from Konstantin Belousov on Mon, 23 Apr 2012 16:03:43 %2B0300) 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> <CAPHpMumh3YpB3RDD-7g5tU6thiuNA6HTuVxmt-9_OzUiEdEXzA@mail.gmail.com> <20120423130343.GT2358@deviant.kiev.zoral.com.ua>
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>>>>> On Mon, 23 Apr 2012 16:03:43 +0300, Konstantin Belousov said: > > On Mon, Apr 23, 2012 at 08:33:05PM +0800, Fengwei yin wrote: > > 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> wrote: > > >> > 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.ru> 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() __asm __volatile("lock; addl $0,(%%esp)" : : : "memory") > > >> >> >> > #define wmb() __asm __volatile("lock; addl $0,(%%esp)" : : : "memory") > > >> >> >> > #define rmb() __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 current > > > thread just set it. > > > > > > I am not sure how much real this race is. Each call to _sinit() is immediately > > > followed by a lock acquire, typically FLOCKFILE(), which enforces full barrier > > > semantic due to pthread_mutex_lock call. The exit() performs __cxa_finalize() > > > 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. > > Yes, FUNLOCKFILE() there would have no effect on the potential CPU reordering > of the writes. But does the order of these writes matter at all ? > > Please note that __sfp() reinitializes all fields written by fclose(). > Only if CPU executing fclose() is allowed to reorder operations so that > the external effect of _flags = 0 assignment can be observed before that > CPU executes other operations from fclose(), there could be a problem. > > This is definitely impossible on Intel, and I indeed do not know about > other architectures enough to reject such possibility. The _flags member > is short, so atomics cannot be used there. The easier solution, if this > is indeed an issue, is to lock thread_lock around _flags = 0 assignment > in fclose(). This can be a problem, even on Intel, because the compiler can reorder the stores. E.g. if I compile the following with gcc -O4 on amd64: struct foo { int x, y; }; int foo(struct foo *p) { int x = bar(); p->y = baz(); p->x = x; } then I get the following assembly language, which sets p->x before p->y: movq %rdi, %rbx call bar movl %eax, %ebp xorl %eax, %eax call baz movl %ebp, (%rbx) movl %eax, 4(%rbx) __Martin
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