About the memory barrier in BSD libc

[Fengwei yin]

On Mon, Apr 23, 2012 at 9:03 PM, Konstantin Belousov
<kostikbel at gmail.com> wrote:
> 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 at 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 at 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 at 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 at 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().

For Intel x86, I suppose it's not a problem because of its memory model.
For IA64, ARM, it could be an issue in theory.

For the solution: I suppose memory barrier is enough for it.