From: Steve Munroe <sjmunroe@us.ibm.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>,
Glibc hackers <libc-hacker@sources.redhat.com>,
rsa@linux.vnet.ibm.com
Subject: Re: [PATCH] Use rel semantics of cas instead of acq semantics with full barrier before it in _int_free
Date: Tue, 14 Jul 2009 14:44:00 -0000 [thread overview]
Message-ID: <OF1490D162.0AE62315-ON862575F3.004F9E63-862575F3.0050EDB0@us.ibm.com> (raw)
In-Reply-To: <20090713161031.GF3101@sunsite.ms.mff.cuni.cz>
Jakub Jelinek <jakub@redhat.com> wrote on 07/13/2009 11:10:31 AM:
> Hi!
>
> The following patch fixes catomic_compare_and_exchange_*_rel definitions
> (which were never used and weren't correct) and uses
> catomic_compare_and_exchange_val_rel in _int_free. Comparing to the
> pre-2009-07-02 --enable-experimental-malloc state the generated code
should
> be identical on all arches other than ppc/ppc64 and on ppc/ppc64 should
use
> lwsync instead of isync barrier.
>
> 2009-07-13 Jakub Jelinek <jakub@redhat.com>
>
> * include/atomic.h (catomic_compare_and_exchange_val_rel): If arch
> overrides atomic_compare_and_exchange_val_rel, define to
> atomic_compare_and_exchange_val_rel by default, otherwise default
> to catomic_compare_and_exchange_val_acq.
> (catomic_compare_and_exchange_bool_rel): If arch overrides
> atomic_compare_and_exchange_bool_rel, define to
> atomic_compare_and_exchange_bool_rel by default.
> * malloc/malloc.c (_int_free): Revert 2009-07-02 change.
> Use catomic_compare_and_exchange_val_rel instead of
> catomic_compare_and_exchange_val_acq.
>
> --- libc/include/atomic.h.jj 2009-05-16 19:23:35.000000000 +0200
> +++ libc/include/atomic.h 2009-07-13 17:47:02.000000000 +0200
> @@ -107,14 +107,19 @@
> #endif
>
>
> -#ifndef atomic_compare_and_exchange_val_rel
> -# define atomic_compare_and_exchange_val_rel(mem, newval, oldval)
\
> - atomic_compare_and_exchange_val_acq (mem, newval, oldval)
> +#ifndef catomic_compare_and_exchange_val_rel
> +# ifndef atomic_compare_and_exchange_val_rel
> +# define catomic_compare_and_exchange_val_rel(mem, newval, oldval)
\
> + catomic_compare_and_exchange_val_acq (mem, newval, oldval)
> +# else
> +# define catomic_compare_and_exchange_val_rel(mem, newval, oldval)
\
> + atomic_compare_and_exchange_val_rel (mem, newval, oldval)
> +# endif
> #endif
>
I assume that defining catomic_compare_and_exchange_val_rel in terms of
catomic_compare_and_exchange_val_acq is only for platforms where there is
no
distinction (between _acq/_rel) and the platforms sysdeps atomic.h did not
define separate *_rel macros.
Otherwise this is a bit confusing...
> snip ...
> --- libc/malloc/malloc.c.jj 2009-07-07 19:10:19.000000000 +0200
> +++ libc/malloc/malloc.c 2009-07-13 17:47:52.000000000 +0200
> @@ -4822,9 +4822,8 @@ _int_free(mstate av, mchunkptr p)
> goto errout;
> }
> p->fd = fd = old;
> - atomic_full_barrier ();
> }
> - while ((old = catomic_compare_and_exchange_val_acq (fb, p, fd)) !=
fd);
> + while ((old = catomic_compare_and_exchange_val_rel (fb, p, fd)) !=
fd);
> #else
> /* Another simple check: make sure the top of the bin is not the
> record we are going to add (i.e., double free). */
>
> Jakub
This is a better solution as the use case matches release semantics. In
this case we are exporting the "p->fd = fd = old" and needs the export/read
barrier that the *_rel macro naturally provides. A import/write barrier is
not required as the do/while iterates only for retry.
For PPC the selection of raw *_acq vs *_rel macros depends on the context.
Thanks
Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center
prev parent reply other threads:[~2009-07-14 14:44 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-07-13 16:11 Jakub Jelinek
2009-07-14 14:44 ` Steve Munroe [this message]
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