[PATCH v3 0/3] perf: Fix SIGCHLD vs pause() race with short-lived workloads

Swapnil Sapkal
2026-04-22T05:06:23Z

Several perf subcommands (sched stats, lock contention) use the pattern
of forking a workload child, calling evlist__start_workload() to uncork
it, and then calling pause() to wait for a signal (typically SIGCHLD
when the child exits, or SIGINT/SIGTERM from the user).

This pattern has a race condition: if the workload is very short-lived,
the child can exit and deliver SIGCHLD in the window between
evlist__start_workload() and pause(). Since pause() only returns when a
signal is received *while the process is suspended*, and SIGCHLD has
already been delivered and handled by the empty sighandler(), pause()
blocks indefinitely.

The fix replaces pause() with a simpler approach:

- When a workload is given (argc), use waitpid() to directly wait for
the child process to exit. This is race-free since waitpid()
collects the child's status regardless of when it exited.

- In system-wide mode (no workload), use 'while (!done) sleep(1)' to
wait for SIGINT/SIGTERM. The signal handler sets a
'volatile sig_atomic_t done' flag, and sleep() is interrupted by
signal delivery so the flag is checked promptly.

Three call sites are affected across two files:
- perf_sched__schedstat_record() in builtin-sched.c
- perf_sched__schedstat_live() in builtin-sched.c
- __cmd_contention() in builtin-lock.c

The two pause() sites in builtin-kwork.c are NOT affected because they
do not register SIGCHLD or fork workload children; they only wait for
user-initiated SIGINT/SIGTERM.

Changes since v2:
- Replaced sigsuspend()/sigprocmask() approach with the much simpler
waitpid() + while(!done) sleep(1) pattern as suggested by
Namhyung. This eliminates all signal mask manipulation while
still being race-free.

Changes since v1:
- Moved sigprocmask() to after evlist__prepare_workload() so the
forked child does not inherit a blocked SIGCHLD mask, which would
break workloads relying on SIGCHLD (Sashiko review)
- Block SIGINT and SIGTERM in addition to SIGCHLD to prevent an
early Ctrl+C during setup from being consumed before sigsuspend().
- Error paths before sigprocmask no longer need mask restoration
since the mask is not yet modified at that point.
(Sashiko review)

Swapnil Sapkal (3):
perf sched stats: Fix SIGCHLD vs pause() race in schedstat_record()
perf sched stats: Fix SIGCHLD vs pause() race in schedstat_live()
perf lock contention: Fix SIGCHLD vs pause() race in __cmd_contention()

tools/perf/builtin-lock.c | 17 +++++++++++++----
tools/perf/builtin-sched.c | 28 ++++++++++++++++++++--------
2 files changed, 33 insertions(+), 12 deletions(-)

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2.43.0