From 672ce9397b78c077e21d785a2ddf9fffd51a1952 Mon Sep 17 00:00:00 2001
From: Richard Henderson <rth@redhat.com>
Date: Thu, 20 Feb 2014 09:43:53 -0800
Subject: [PATCH] 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
---
gcc/ChangeLog | 6 ++++++
gcc/builtins.c | 19 ++++++++++++++-----
libatomic/ChangeLog | 6 ++++++
libatomic/cas_n.c | 25 ++++++++++++-------------
4 files changed, 38 insertions(+), 18 deletions(-)
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9333fb6804c7..7161754b2b3d 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -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>
diff --git a/gcc/builtins.c b/gcc/builtins.c
index f5f55bf0e2e6..09fefe50a8d9 100644
--- a/gcc/builtins.c
+++ b/gcc/builtins.c
@@ -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;
}
diff --git a/libatomic/ChangeLog b/libatomic/ChangeLog
index 2d1c9bd18ae7..1bd3c2178aa5 100644
--- a/libatomic/ChangeLog
+++ b/libatomic/ChangeLog
@@ -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
diff --git a/libatomic/cas_n.c b/libatomic/cas_n.c
index 39c78332423c..801262d551c4 100644
--- a/libatomic/cas_n.c
+++ b/libatomic/cas_n.c
@@ -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);
--
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