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charset="utf-8" If the profiled workload exits very quickly, SIGCHLD can be delivered and consumed by the empty signal handler before the process enters pause(), causing an indefinite hang. Fix this with a simpler approach: - The signal handler now sets a 'volatile sig_atomic_t done' flag. Reset 'done' before registering signal handlers so that an early signal during setup is not discarded by a later reset. - Replace pause() with a loop that checks 'done' and uses waitpid(WNOHANG) to detect child exit without blocking. This handles both workload mode (child exits) and system-wide mode (user sends SIGINT/SIGTERM). Using WNOHANG avoids the SA_RESTART problem where a blocking waitpid() would auto-restart and ignore the done flag if the child doesn't exit on signal. Suggested-by: Namhyung Kim Assisted-by: Claude:claude-opus-4.6 Signed-off-by: Swapnil Sapkal --- tools/perf/builtin-sched.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c index 53a93aa18853..7da71c372e25 100644 --- a/tools/perf/builtin-sched.c +++ b/tools/perf/builtin-sched.c @@ -36,6 +36,7 @@ #include #include #include +#include #include =20 #include @@ -3758,8 +3759,11 @@ static int process_synthesized_schedstat_event(const= struct perf_tool *tool, return 0; } =20 +static volatile sig_atomic_t done; + static void sighandler(int sig __maybe_unused) { + done =3D 1; } =20 static int enable_sched_schedstats(int *reset) @@ -3819,6 +3823,7 @@ static int perf_sched__schedstat_record(struct perf_s= ched *sched, .mode =3D PERF_DATA_MODE_WRITE, }; =20 + done =3D 0; signal(SIGINT, sighandler); signal(SIGCHLD, sighandler); signal(SIGTERM, sighandler); @@ -3903,8 +3908,11 @@ static int perf_sched__schedstat_record(struct perf_= sched *sched, if (argc) evlist__start_workload(evlist); 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charset="utf-8" perf_sched__schedstat_live() has the same lost-wakeup race as perf_sched__schedstat_record(): a short-lived workload's SIGCHLD can be consumed by the signal handler before pause() is entered, hanging the process. Apply the same fix: replace pause() with a loop checking the 'done' flag and using waitpid(WNOHANG) for the workload case. Suggested-by: Namhyung Kim Assisted-by: Claude:claude-opus-4.6 Signed-off-by: Swapnil Sapkal --- tools/perf/builtin-sched.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c index 7da71c372e25..9e8941ae1b3d 100644 --- a/tools/perf/builtin-sched.c +++ b/tools/perf/builtin-sched.c @@ -4755,6 +4755,7 @@ static int perf_sched__schedstat_live(struct perf_sch= ed *sched, int reset =3D 0; int err =3D 0; =20 + done =3D 0; signal(SIGINT, sighandler); signal(SIGCHLD, sighandler); signal(SIGTERM, sighandler); @@ -4800,8 +4801,11 @@ static int perf_sched__schedstat_live(struct perf_sc= hed *sched, if (argc) evlist__start_workload(evlist); =20 - /* wait for signal */ - pause(); + while (!done) { + if (argc && waitpid(evlist->workload.pid, NULL, WNOHANG) > 0) + break; + sleep(1); + } =20 if (reset) { err =3D disable_sched_schedstat(); --=20 2.43.0 From nobody Mon May 25 00:08:42 2026 Received: from BN1PR04CU002.outbound.protection.outlook.com (mail-eastus2azon11010058.outbound.protection.outlook.com [52.101.56.58]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8CD943403E2; 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Wed, 20 May 2026 05:21:50 -0500 From: Swapnil Sapkal To: , , , , , CC: , , , , , , , "Swapnil Sapkal" Subject: [PATCH v4 3/3] perf lock contention: Fix SIGCHLD vs pause() race in __cmd_contention() Date: Wed, 20 May 2026 10:20:17 +0000 Message-ID: <20260520102017.293419-4-swapnil.sapkal@amd.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260520102017.293419-1-swapnil.sapkal@amd.com> References: <20260520102017.293419-1-swapnil.sapkal@amd.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: satlexmb07.amd.com (10.181.42.216) To satlexmb07.amd.com (10.181.42.216) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SA2PEPF00003F65:EE_|SA1PR12MB9515:EE_ X-MS-Office365-Filtering-Correlation-Id: cdf22454-7916-4d2e-f238-08deb6599d97 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|36860700016|1800799024|82310400026|18002099003|56012099003|22082099003|11063799006; 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charset="utf-8" __cmd_contention() suffers from the same lost-wakeup race as the perf sched stats paths: SIGCHLD can be consumed by the signal handler before pause() is entered, hanging the process. Apply the same fix: replace pause() with a loop checking the 'done' flag and using waitpid(WNOHANG) for the workload case. Suggested-by: Namhyung Kim Assisted-by: Claude:claude-opus-4.6 Signed-off-by: Swapnil Sapkal --- tools/perf/builtin-lock.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c index 5585aeb97684..e1c033e3a8d5 100644 --- a/tools/perf/builtin-lock.c +++ b/tools/perf/builtin-lock.c @@ -31,6 +31,7 @@ #include #include #include +#include #include #include #include @@ -1921,8 +1922,11 @@ static int __cmd_report(bool display_info) return err; } =20 +static volatile sig_atomic_t done; + static void sighandler(int sig __maybe_unused) { + done =3D 1; } =20 static int check_lock_contention_options(const struct option *options, @@ -2060,6 +2064,7 @@ static int __cmd_contention(int argc, const char **ar= gv) goto out_delete; } =20 + done =3D 0; signal(SIGINT, sighandler); signal(SIGCHLD, sighandler); signal(SIGTERM, sighandler); @@ -2127,8 +2132,11 @@ static int __cmd_contention(int argc, const char **a= rgv) if (argc) evlist__start_workload(con.evlist); =20 - /* wait for signal */ - pause(); + while (!done) { + if (argc && waitpid(con.evlist->workload.pid, NULL, WNOHANG) > 0) + break; + sleep(1); + } =20 lock_contention_stop(); lock_contention_read(&con); --=20 2.43.0