[PATCH] Fix SIGSTOP locking issue in -rt

["Paul E. McKenney" <paulmck@us.ibm.com>]

Hello!

The following patch fixes a locking issue in -rt signals, where the
job-control signals drop sighand_lock momentarily during signal handling.
This interacts badly with the RCU-ification of unicast signal delivery.
The fix is simply to acquire tasklist_lock for job-control signals, since
these typically do not have stringent scalability or latency requirements.

These same changes would be useful in recent -mm.

Signed-off-by: <paulmck@us.ibm.com>

---

 signal.c |    8 ++++++++
 1 files changed, 8 insertions(+)

diff -urpNa -X dontdiff linux-2.6.14-rc5-rt2-ckhandRCUfix/kernel/signal.c linux-2.6.14-rc5-rt2-SIGSTOP/kernel/signal.c
--- linux-2.6.14-rc5-rt2-ckhandRCUfix/kernel/signal.c	2005-10-31 22:28:45.000000000 -0800
+++ linux-2.6.14-rc5-rt2-SIGSTOP/kernel/signal.c	2005-11-01 09:32:29.000000000 -0800
@@ -1207,13 +1207,21 @@ int
 kill_proc_info(int sig, struct siginfo *info, pid_t pid)
 {
 	int error;
+	int acquired_tasklist_lock = 0;
 	struct task_struct *p;
 
 	rcu_read_lock();
+	if (unlikely(sig_kernel_stop(sig) || sig == SIGCONT)) {
+		read_lock(&tasklist_lock);
+		acquired_tasklist_lock = 1;
+	}
 	p = find_task_by_pid(pid);
 	error = -ESRCH;
 	if (p)
 		error = group_send_sig_info(sig, info, p);
+	if (unlikely(acquired_tasklist_lock)) {
+		read_unlock(&tasklist_lock);
+	}
 	rcu_read_unlock();
 	return error;
 }