{"id":2226305,"url":"http://patchwork.ozlabs.org/api/1.2/patches/2226305/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.5@forge-stage.sourceware.org/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.2/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"","list_archive_url":null,"date":"2026-04-22T10:44:20","name":"[v1,05/16] libstdc++: Fix race condition in new atomic notify code","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"74f31fa549e2c7039b0733022b828e495abf6ea7","submitter":{"id":93210,"url":"http://patchwork.ozlabs.org/api/1.2/people/93210/?format=json","name":"Jonathan Wakely via Sourceware Forge","email":"forge-bot+redi@forge-stage.sourceware.org"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.5@forge-stage.sourceware.org/mbox/","series":[{"id":500987,"url":"http://patchwork.ozlabs.org/api/1.2/series/500987/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=500987","date":"2026-04-22T10:44:17","name":"atomic wait/notify ABI stabilization","version":1,"mbox":"http://patchwork.ozlabs.org/series/500987/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226305/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226305/checks/","tags":{},"related":[],"headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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server2.sourceware.org","From":"Jonathan Wakely via Sourceware Forge\n ","Date":"Wed, 22 Apr 2026 10:44:20 +0000","Subject":"[PATCH v1 05/16] libstdc++: Fix race condition in new atomic notify\n code","To":"gcc-patches mailing list ","Message-ID":"\n ","X-Mailer":"batrachomyomachia","X-Pull-Request-Organization":"gcc","X-Pull-Request-Repository":"gcc-TEST","X-Pull-Request":"https://forge.sourceware.org/gcc/gcc-TEST/pulls/31","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/redi/gcc/commit/b7703d2c63548243f4f85f4393e0aff3ec89ed6a","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"gcc-patches mailing list , redi@gcc.gnu.org","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"From: Jonathan Wakely \n\nWhen using a proxy object for atomic waiting and notifying operations,\nwe need to ensure that the _M_ver value is always incremented by a\nnotifying operation, even if we return early without doing the futex\nwake syscall. Otherwise we get missed wake-ups because the notifying\nthread doesn't modify the value that other threads are doing a futex\nwait on.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/atomic_wait.h (__notify_impl): Increment the\n\tproxy value before returning early for the uncontended case.\n---\n libstdc++-v3/include/bits/atomic_wait.h | 18 +++++++++---------\n 1 file changed, 9 insertions(+), 9 deletions(-)","diff":"diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h\nindex 172910a4e34f..725ea030245d 100644\n--- a/libstdc++-v3/include/bits/atomic_wait.h\n+++ b/libstdc++-v3/include/bits/atomic_wait.h\n@@ -406,18 +406,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n {\n __waiter_pool_impl* __pool = nullptr;\n \n- if (__args & __wait_flags::__track_contention)\n-\t{\n-\t __pool = &__waiter_pool_impl::_S_impl_for(__addr);\n-\t if (!__pool->_M_waiting())\n-\t return;\n-\t}\n-\n const __platform_wait_t* __wait_addr;\n if (__args & __wait_flags::__proxy_wait)\n \t{\n-\t if (!__pool)\n-\t __pool = &__waiter_pool_impl::_S_impl_for(__addr);\n+\t __pool = &__waiter_pool_impl::_S_impl_for(__addr);\n \t // Waiting for *__addr is actually done on the proxy's _M_ver.\n \t __wait_addr = &__pool->_M_ver;\n \t __atomic_fetch_add(&__pool->_M_ver, 1, __ATOMIC_RELAXED);\n@@ -430,6 +422,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n else // Use the atomic variable's own address.\n \t__wait_addr = __addr;\n \n+ if (__args & __wait_flags::__track_contention)\n+\t{\n+\t if (!__pool)\n+\t __pool = &__waiter_pool_impl::_S_impl_for(__addr);\n+\t if (!__pool->_M_waiting())\n+\t return;\n+\t}\n+\n #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT\n __platform_notify(__wait_addr, __all);\n #else\n","prefixes":["v1","05/16"]}