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

[Swapnil Sapkal]

Some 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 uses the standard POSIX pattern for this class of bug:

1. Block SIGCHLD (via sigprocmask) before starting the workload.
   If the child exits, the signal remains pending rather than being
   delivered and lost.

2. Replace pause() with sigsuspend(&oldmask), which atomically
   unblocks SIGCHLD and suspends the process. There is no window
   where the signal can slip through unnoticed.

3. Restore the original signal mask after sigsuspend() returns.

SIGINT and SIGTERM are not blocked at any point, so Ctrl+C and
graceful termination continue to work exactly as before.

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.

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

 tools/perf/builtin-lock.c  | 20 ++++++++++++++++++--
 tools/perf/builtin-sched.c | 30 ++++++++++++++++++++++++++----
 2 files changed, 44 insertions(+), 6 deletions(-)

-- 
2.43.0