* [PATCH] Use rel semantics of cas instead of acq semantics with full barrier before it in _int_free
@ 2009-07-13 16:11 Jakub Jelinek
2009-07-14 14:44 ` Steve Munroe
0 siblings, 1 reply; 2+ messages in thread
From: Jakub Jelinek @ 2009-07-13 16:11 UTC (permalink / raw)
To: Ulrich Drepper, Steve Munroe, Ryan Arnold; +Cc: Glibc hackers
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
-#ifndef catomic_compare_and_exchange_val_rel
-# define catomic_compare_and_exchange_val_rel(mem, newval, oldval) \
+#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)
#endif
@@ -155,15 +160,20 @@
#endif
-#ifndef atomic_compare_and_exchange_bool_rel
-# define atomic_compare_and_exchange_bool_rel(mem, newval, oldval) \
- atomic_compare_and_exchange_bool_acq (mem, newval, oldval)
+#ifndef catomic_compare_and_exchange_bool_rel
+# ifndef atomic_compare_and_exchange_bool_rel
+# define catomic_compare_and_exchange_bool_rel(mem, newval, oldval) \
+ catomic_compare_and_exchange_bool_acq (mem, newval, oldval)
+# else
+# define catomic_compare_and_exchange_bool_rel(mem, newval, oldval) \
+ atomic_compare_and_exchange_bool_rel (mem, newval, oldval)
+# endif
#endif
-#ifndef catomic_compare_and_exchange_bool_rel
-# define catomic_compare_and_exchange_bool_rel(mem, newval, oldval) \
- catomic_compare_and_exchange_bool_acq (mem, newval, oldval)
+#ifndef atomic_compare_and_exchange_bool_rel
+# define atomic_compare_and_exchange_bool_rel(mem, newval, oldval) \
+ atomic_compare_and_exchange_bool_acq (mem, newval, oldval)
#endif
--- 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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] Use rel semantics of cas instead of acq semantics with full barrier before it in _int_free
2009-07-13 16:11 [PATCH] Use rel semantics of cas instead of acq semantics with full barrier before it in _int_free Jakub Jelinek
@ 2009-07-14 14:44 ` Steve Munroe
0 siblings, 0 replies; 2+ messages in thread
From: Steve Munroe @ 2009-07-14 14:44 UTC (permalink / raw)
To: Jakub Jelinek; +Cc: Ulrich Drepper, Glibc hackers, rsa
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
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2009-07-14 14:44 ` Steve Munroe
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