From 9bdade1bc13e547130d2629291758a579722e5d1 Mon Sep 17 00:00:00 2001

From: Oleg Nesterov <oleg@redhat.com>

Date: Fri, 5 Nov 2010 16:53:42 +0100

Subject: posix-cpu-timers: workaround to suppress the problems with mt exec

posix-cpu-timers.c correctly assumes that the dying process does

posix_cpu_timers_exit_group() and removes all !CPUCLOCK_PERTHREAD

timers from signal->cpu_timers list.

But, it also assumes that timer->it.cpu.task is always the group

leader, and thus the dead ->task means the dead thread group.

This is obviously not true after de_thread() changes the leader.

After that almost every posix_cpu_timer_ method has problems.

It is not simple to fix this bug correctly. First of all, I think

that timer->it.cpu should use struct pid instead of task_struct.

Also, the locking should be reworked completely. In particular,

tasklist_lock should not be used at all. This all needs a lot of

nontrivial and hard-to-test changes.

Change __exit_signal() to do posix_cpu_timers_exit_group() when

the old leader dies during exec. This is not the fix, just the

temporary hack to hide the problem for 2.6.37 and stable. IOW,

this is obviously wrong but this is what we currently have anyway:

cpu timers do not work after mt exec.

In theory this change adds another race. The exiting leader can

detach the timers which were attached to the new leader. However,

the window between de_thread() and release_task() is small, we

can pretend that sys_timer_create() was called before de_thread().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

---

 kernel/exit.c |    8 ++++++++

 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c

index 45102e9..02b7104 100644

--- a/kernel/exit.c

+++ b/kernel/exit.c

@@ -92,6 +92,14 @@ static void __exit_signal(struct task_struct *tsk)

 		posix_cpu_timers_exit_group(tsk);

 	else {

 		/*

+		 * This can only happen if the caller is de_thread().

+		 * FIXME: this is the temporary hack, we should teach

+		 * posix-cpu-timers to handle this case correctly.

+		 */

+		if (unlikely(has_group_leader_pid(tsk)))

+			posix_cpu_timers_exit_group(tsk);

+

+		/*

 		 * If there is any task waiting for the group exit

 		 * then notify it:

 		 */

-- 

1.7.3.2