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[wasm][gc] Use acquire-release semantics for ref count

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TSan reports errors if one thread changes the ref count using relaxed
semantics, then another thread frees the code object. Acquire-release
semantics fix this, as they impose an ordering between the memory
accesses of different threads.

R=mstarzinger@chromium.org

Bug: v8:8217, v8:9200
Change-Id: I30ce150154e6459c2c64e16be603f29187af1dcd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1594553
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61205}
This commit is contained in:
Clemens Hammacher 2019-05-03 11:52:20 +02:00 committed by Commit Bot
parent 9a69ae3d6e
commit 02ebf5879f
1 changed files with 4 additions and 4 deletions
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8
src/wasm/wasm-code-manager.h
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@ -148,7 +148,7 @@ class V8_EXPORT_PRIVATE WasmCode final {
~WasmCode();
void IncRef() {
int old_val = ref_count_.fetch_add(1, std::memory_order_relaxed);
int old_val = ref_count_.fetch_add(1, std::memory_order_acq_rel);
DCHECK_LE(1, old_val);
DCHECK_GT(kMaxInt, old_val);
USE(old_val);
@ -157,12 +157,12 @@ class V8_EXPORT_PRIVATE WasmCode final {
// Decrement the ref count. Returns whether this code becomes dead and needs
// to be freed.
V8_WARN_UNUSED_RESULT bool DecRef() {
int old_count = ref_count_.load(std::memory_order_relaxed);
int old_count = ref_count_.load(std::memory_order_acquire);
while (true) {
DCHECK_LE(1, old_count);
if (V8_UNLIKELY(old_count == 1)) return DecRefOnPotentiallyDeadCode();
if (ref_count_.compare_exchange_weak(old_count, old_count - 1,
std::memory_order_relaxed)) {
std::memory_order_acq_rel)) {
return false;
}
}
@ -172,7 +172,7 @@ class V8_EXPORT_PRIVATE WasmCode final {
// might still be C++ references. Returns whether this drops the last
// reference and the code needs to be freed.
V8_WARN_UNUSED_RESULT bool DecRefOnDeadCode() {
return ref_count_.fetch_sub(1, std::memory_order_relaxed) == 1;
return ref_count_.fetch_sub(1, std::memory_order_acq_rel) == 1;
}
// Decrement the ref count on a set of {WasmCode} objects, potentially