linux-kernel - Re: rcu_preempt detected stalls.

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Message-ID: <20141023202443.GE4977@linux.vnet.ibm.com>
Date:	Thu, 23 Oct 2014 13:24:44 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, htejun@...il.com
Subject: Re: rcu_preempt detected stalls.

On Thu, Oct 23, 2014 at 09:53:37PM +0200, Oleg Nesterov wrote:
> On 10/23, Paul E. McKenney wrote:
> >
> > OK, so making each pass through the loop a separate RCU read-side critical
> > section might be considered to be suppressing notification of an error
> > condition?
> 
> I agree, this change probably makes sense anyway. Personally I'd prefer
> the version below (somehow I hate multiple unlock's), but I won't insist.

Your code, your rules.  ;-)

But given this structure, why not use a for() loop replace the
"goto retry" with an inverted condition and a "return error"?
Maybe something like the following patch?

							Thanx, Paul

------------------------------------------------------------------------

signal: Exit RCU read-side critical section on each pass through loop

The kill_pid_info() can potentially loop indefinitely if tasks are created
and deleted sufficiently quickly, and if this happens, this function
will remain in a single RCU read-side critical section indefinitely.
This commit therefore exits the RCU read-side critical section on each
pass through the loop.  Because a race must happen to retry the loop,
this should have no performance impact in the common case.

Reported-by: Dave Jones <davej@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@...hat.com>

diff --git a/kernel/signal.c b/kernel/signal.c
index 8f0876f9f6dd..54820984a872 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1331,23 +1331,21 @@ int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
 	int error = -ESRCH;
 	struct task_struct *p;
 
-	rcu_read_lock();
-retry:
-	p = pid_task(pid, PIDTYPE_PID);
-	if (p) {
-		error = group_send_sig_info(sig, info, p);
-		if (unlikely(error == -ESRCH))
-			/*
-			 * The task was unhashed in between, try again.
-			 * If it is dead, pid_task() will return NULL,
-			 * if we race with de_thread() it will find the
-			 * new leader.
-			 */
-			goto retry;
-	}
-	rcu_read_unlock();
+	for (;;) {
+		rcu_read_lock();
+		p = pid_task(pid, PIDTYPE_PID);
+		if (p)
+			error = group_send_sig_info(sig, info, p);
+		rcu_read_unlock();
+		if (likely(!p || error != -ESRCH))
+			return error;
 
-	return error;
+		/*
+		 * The task was unhashed in between, try again.  If it
+		 * is dead, pid_task() will return NULL, if we race with
+		 * de_thread() it will find the new leader.
+		 */
+	}
 }
 
 int kill_proc_info(int sig, struct siginfo *info, pid_t pid)

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