From: Rusty Russell <rusty@rustcorp.com.au>
Ingo recently accidentally broke CPU hotplug by enabling preemption around
release_task(), which can be called on the current task if the parent isn't
interested.
The problem is, the task can be preempted and then the CPU can go down:
it's not in the task list any more, and so it won't get migrated after the
CPU goes down. It stays on the down CPU, which triggers a BUG_ON.
We have had previous problems with tasks releasing themselves: oprofile has
a comment about it, and we had the case of trying to deliver SIGXCPU in the
timer tick to the current task which had called release_task(). This patch
shuffles the self-reaping off to finish_task_switch, so there's never a
running task which isn't in the task list, except idle threads.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (authored)
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/include/linux/sched.h | 1 +
25-akpm/kernel/exit.c | 28 +++++++---------------------
25-akpm/kernel/sched.c | 5 ++++-
3 files changed, 12 insertions(+), 22 deletions(-)
diff -puN include/linux/sched.h~dont-sleep-after-were-out-of-task-list include/linux/sched.h
--- 25/include/linux/sched.h~dont-sleep-after-were-out-of-task-list 2004-08-17 23:56:04.674166560 -0700
+++ 25-akpm/include/linux/sched.h 2004-08-17 23:56:04.681165496 -0700
@@ -565,6 +565,7 @@ do { if (atomic_dec_and_test(&(tsk)->usa
#define PF_STARTING 0x00000002 /* being created */
#define PF_EXITING 0x00000004 /* getting shut down */
#define PF_DEAD 0x00000008 /* Dead */
+#define PF_SELFREAP 0x00000010 /* Never a zombie, must be released */
#define PF_FORKNOEXEC 0x00000040 /* forked but didn't exec */
#define PF_SUPERPRIV 0x00000100 /* used super-user privileges */
#define PF_DUMPCORE 0x00000200 /* dumped core */
diff -puN kernel/exit.c~dont-sleep-after-were-out-of-task-list kernel/exit.c
--- 25/kernel/exit.c~dont-sleep-after-were-out-of-task-list 2004-08-17 23:56:04.675166408 -0700
+++ 25-akpm/kernel/exit.c 2004-08-17 23:56:04.682165344 -0700
@@ -755,8 +755,8 @@ static void exit_notify(struct task_stru
state = TASK_ZOMBIE;
if (tsk->exit_signal == -1 && tsk->ptrace == 0)
state = TASK_DEAD;
- else
- tsk->state = state;
+ tsk->state = state;
+
/*
* Clear these here so that update_process_times() won't try to deliver
* itimer, profile or rlimit signals to this task while it is in late exit.
@@ -765,14 +765,6 @@ static void exit_notify(struct task_stru
tsk->it_prof_value = 0;
tsk->rlim[RLIMIT_CPU].rlim_cur = RLIM_INFINITY;
- /*
- * Get a reference to it so that we can set the state
- * as the last step. The state-setting only matters if the
- * current task is releasing itself, to trigger the final
- * put_task_struct() in finish_task_switch(). (thread self-reap)
- */
- get_task_struct(tsk);
-
write_unlock_irq(&tasklist_lock);
list_for_each_safe(_p, _n, &ptrace_dead) {
@@ -781,18 +773,12 @@ static void exit_notify(struct task_stru
release_task(t);
}
- /* If the process is dead, release it - nobody will wait for it */
- if (state == TASK_DEAD) {
- release_task(tsk);
- write_lock_irq(&tasklist_lock);
- tsk->state = state;
- _raw_write_unlock(&tasklist_lock);
- local_irq_enable();
- } else
- preempt_disable();
-
+ preempt_disable();
+ /* PF_DEAD says drop ref after we schedule. */
tsk->flags |= PF_DEAD;
- put_task_struct(tsk);
+ /* PF_SELFREAP says there's no parent to wait4() for us. */
+ if (state == TASK_DEAD)
+ tsk->flags |= PF_SELFREAP;
}
asmlinkage NORET_TYPE void do_exit(long code)
diff -puN kernel/sched.c~dont-sleep-after-were-out-of-task-list kernel/sched.c
--- 25/kernel/sched.c~dont-sleep-after-were-out-of-task-list 2004-08-17 23:56:04.677166104 -0700
+++ 25-akpm/kernel/sched.c 2004-08-17 23:56:04.685164888 -0700
@@ -1484,8 +1484,11 @@ static void finish_task_switch(task_t *p
finish_arch_switch(rq, prev);
if (mm)
mmdrop(mm);
- if (unlikely(prev_task_flags & PF_DEAD))
+ if (unlikely(prev_task_flags & PF_DEAD)) {
+ if (prev_task_flags & PF_SELFREAP)
+ release_task(prev);
put_task_struct(prev);
+ }
}
/**
_