Re: PR/49816 CVS commit: src/libexec/ld.elf_so

[yamt%netbsd.org@localhost (YAMAMOTO Takashi)]

> The following reply was made to PR lib/49816; it has been noted by GNATS.
> 
> From: Joerg Sonnenberger <joerg%britannica.bec.de@localhost>
> To: gnats-bugs%NetBSD.org@localhost
> Cc: 
> Subject: Re: PR/49816 CVS commit: src/libexec/ld.elf_so
> Date: Mon, 6 Apr 2015 20:16:22 +0200
> 
>  On Mon, Apr 06, 2015 at 04:45:01PM +0000, YAMAMOTO Takashi wrote:
>  > The following reply was made to PR lib/49816; it has been noted by GNATS.
>  > 
>  > From: yamt%netbsd.org@localhost (YAMAMOTO Takashi)
>  > To: gnats-bugs%NetBSD.org@localhost
>  > Cc: lib-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost,
>  > 	yamt%NetBSD.org@localhost
>  > Subject: Re: PR/49816 CVS commit: src/libexec/ld.elf_so
>  > Date: Mon,  6 Apr 2015 16:41:01 +0000 (UTC)
>  > 
>  >  > The following reply was made to PR lib/49816; it has been noted by GNATS.
>  >  > 
>  >  > From: Joerg Sonnenberger <joerg%britannica.bec.de@localhost>
>  >  > To: gnats-bugs%NetBSD.org@localhost
>  >  > Cc: lib-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
>  >  > 	netbsd-bugs%netbsd.org@localhost, yamt%NetBSD.org@localhost
>  >  > Subject: Re: PR/49816 CVS commit: src/libexec/ld.elf_so
>  >  > Date: Mon, 6 Apr 2015 18:22:21 +0200
>  >  > 
>  >  >  On Mon, Apr 06, 2015 at 04:10:02PM +0000, YAMAMOTO Takashi wrote:
>  >  >  >  except that:
>  >  >  >  * _rtld_exclusive_exit doesn't use CAS
>  >  >  >  * this code is MI
>  >  >  >  
>  >  >  >  i agree that something like PTHREAD__ATOMIC_IS_MEMBAR
>  >  >  >  would be a nice optimization, though.
>  >  >  
>  >  >  So which platform are you worried about that doesn't have TSO and
>  >  >  doesn't implicit membars for CAS? I'm asking because the only reason
>  >  >  those changes should help your problem is if they massively penalize the
>  >  >  operation.
>  >  
>  >  well, can you explain why _rtld_exclusive_exit is safe
>  >  without cas or barrier?
>  
>  All sane MP platforms at least implement Total Store Ordering. So all
>  unrelated stores are visible no later than the reset of the mutex.
>  That's why I am surprised that it changes anything at all for you.
>  
>  Joerg

the intel's manual says:  (8.2.2)

    Reads may be reordered with older writes to different locations
    but not with older writes to the same location.

see also 8.2.3.4 Example 8-3.

isn't it the case for _rtld_exclusive_exit?

YAMAMOTO Takashi