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re PR c++/60272 (atomic<>::compare_exchange_weak has spurious store and can cause race conditions) 

PR c++/60272

gcc/
	* builtins.c (expand_builtin_atomic_compare_exchange): Conditionalize
	on failure the store back into EXPECT.
libatomic/
	* cas_n.c (libat_compare_exchange): Conditionalize on failure
	the store back to EPTR.

From-SVN: r207966
This commit is contained in:
Richard Henderson 2014-02-20 09:43:53 -08:00 committed by Richard Henderson
parent 95ce7613d5
commit 672ce9397b
4 changed files with 38 additions and 18 deletions
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6
gcc/ChangeLog
View File

@ -1,3 +1,9 @@
2014-02-20 Richard Henderson <rth@redhat.com>
PR c++/60272
* builtins.c (expand_builtin_atomic_compare_exchange): Conditionalize
on failure the store back into EXPECT.
2014-02-20 Chung-Lin Tang <cltang@codesourcery.com>
Sandra Loosemore <sandra@codesourcery.com>

19
gcc/builtins.c
View File

@ -5292,7 +5292,7 @@ static rtx
expand_builtin_atomic_compare_exchange (enum machine_mode mode, tree exp,
rtx target)
{
rtx expect, desired, mem, oldval;
rtx expect, desired, mem, oldval, label;
enum memmodel success, failure;
tree weak;
bool is_weak;
@ -5330,14 +5330,23 @@ expand_builtin_atomic_compare_exchange (enum machine_mode mode, tree exp,
if (tree_fits_shwi_p (weak) && tree_to_shwi (weak) != 0)
is_weak = true;
if (target == const0_rtx)
target = NULL;
oldval = expect;
if (!expand_atomic_compare_and_swap ((target == const0_rtx ? NULL : &target),
&oldval, mem, oldval, desired,
if (!expand_atomic_compare_and_swap (&target, &oldval, mem, oldval, desired,
is_weak, success, failure))
return NULL_RTX;
if (oldval != expect)
emit_move_insn (expect, oldval);
/* Conditionally store back to EXPECT, lest we create a race condition
with an improper store to memory. */
/* ??? With a rearrangement of atomics at the gimple level, we can handle
the normal case where EXPECT is totally private, i.e. a register. At
which point the store can be unconditional. */
label = gen_label_rtx ();
emit_cmp_and_jump_insns (target, const0_rtx, NE, NULL, VOIDmode, 1, label);
emit_move_insn (expect, oldval);
emit_label (label);
return target;
}

6
libatomic/ChangeLog
View File

@ -1,3 +1,9 @@
2014-02-20 Richard Henderson <rth@redhat.com>
PR c++/60272
* cas_n.c (libat_compare_exchange): Conditionalize on failure
the store back to EPTR.
2014-01-02 Richard Sandiford <rdsandiford@googlemail.com>
Update copyright years

25
libatomic/cas_n.c
View File

@ -51,10 +51,9 @@ SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
#if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
bool
SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
int smodel, int fmodel UNUSED)
int smodel, int fmodel)
{
UWORD mask, shift, weval, woldval, wnewval, t, *wptr;
bool ret = false;
pre_barrier (smodel);
@ -82,12 +81,13 @@ SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
}
while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
__ATOMIC_RELAXED, __ATOMIC_RELAXED));
ret = true;
post_barrier (smodel);
return true;
failure:
*eptr = woldval >> shift;
post_barrier (smodel);
return ret;
post_barrier (fmodel);
return false;
}
#define DONE 1
@ -102,18 +102,17 @@ SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
{
UTYPE oldval;
UWORD magic;
bool ret = false;
bool ret;
pre_seq_barrier (smodel);
magic = protect_start (mptr);
oldval = *mptr;
if (oldval == *eptr)
{
*mptr = newval;
ret = true;
}
*eptr = oldval;
ret = (oldval == *eptr);
if (ret)
*mptr = newval;
else
*eptr = oldval;
protect_end (mptr, magic);
post_seq_barrier (smodel);