Re: rcu_preempt detected stalls.

From: Paul E. McKenney
Date: Thu Oct 23 2014 - 16:28:44 EST


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@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>

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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