Messages in this thread Patch in this message | | | Date | Fri, 18 Jun 2010 21:34:03 +0200 | From | Oleg Nesterov <> | Subject | while_each_thread() under rcu_read_lock() is broken? |
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Hmm. Once I sent this patch, I suddenly realized with horror that while_each_thread() is NOT safe under rcu_read_lock(). Both do_each_thread/while_each_thread or do/while_each_thread() can race with exec().
Yes, it is safe to do next_thread() or next_task(). But:
#define while_each_thread(g, t) \ while ((t = next_thread(t)) != g)
suppose that t is not the group leader, and it does de_thread() and then release_task(g). After that next_thread(t) returns t, not g, and the loop will never stop.
I _really_ hope I missed something, will recheck tomorrow with the fresh head. Still I'd like to share my concerns...
If I am right, probably we can fix this, something like
#define while_each_thread(g, t) \ while ((t = next_thread(t)) != g && pid_alive(g))
[we can't do while (!thread_group_leadr(t = next_thread(t)))]. but this needs barrires, and we should validate the callers anyway.
Or, perhaps,
#define XXX(t) ({ struct task_struct *__prev = t; t = next_thread(t); t != g && t != __prev; })
#define while_each_thread(g, t) \ while (XXX(t))
Please tell me I am wrong!
Oleg.
On 06/18, Oleg Nesterov wrote: > > check_hung_uninterruptible_tasks()->rcu_lock_break() introduced by > "softlockup: check all tasks in hung_task" commit ce9dbe24 looks > absolutely wrong. > > - rcu_lock_break() does put_task_struct(). If the task has exited > it is not safe to even read its ->state, nothing protects this > task_struct. > > - The TASK_DEAD checks are wrong too. Contrary to the comment, we > can't use it to check if the task was unhashed. It can be unhashed > without TASK_DEAD, or it can be valid with TASK_DEAD. > > For example, an autoreaping task can do release_task(current) > long before it sets TASK_DEAD in do_exit(). > > Or, a zombie task can have ->state == TASK_DEAD but release_task() > was not called, and in this case we must not break the loop. > > Change this code to check pid_alive() instead, and do this before we > drop the reference to the task_struct. > > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > > kernel/hung_task.c | 11 +++++++---- > 1 file changed, 7 insertions(+), 4 deletions(-) > > --- 35-rc2/kernel/hung_task.c~CHT_FIX_RCU_LOCK_BREAK 2009-12-18 19:05:38.000000000 +0100 > +++ 35-rc2/kernel/hung_task.c 2010-06-18 20:06:11.000000000 +0200 > @@ -113,15 +113,20 @@ static void check_hung_task(struct task_ > * For preemptible RCU it is sufficient to call rcu_read_unlock in order > * exit the grace period. For classic RCU, a reschedule is required. > */ > -static void rcu_lock_break(struct task_struct *g, struct task_struct *t) > +static bool rcu_lock_break(struct task_struct *g, struct task_struct *t) > { > + bool can_cont; > + > get_task_struct(g); > get_task_struct(t); > rcu_read_unlock(); > cond_resched(); > rcu_read_lock(); > + can_cont = pid_alive(g) && pid_alive(t); > put_task_struct(t); > put_task_struct(g); > + > + return can_cont; > } > > /* > @@ -148,9 +153,7 @@ static void check_hung_uninterruptible_t > goto unlock; > if (!--batch_count) { > batch_count = HUNG_TASK_BATCHING; > - rcu_lock_break(g, t); > - /* Exit if t or g was unhashed during refresh. */ > - if (t->state == TASK_DEAD || g->state == TASK_DEAD) > + if (!rcu_lock_break(g, t)) > goto unlock; > } > /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
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