Messages in this thread Patch in this message | | | Date | Fri, 18 Jun 2010 21:02:51 +0200 | From | Oleg Nesterov <> | Subject | [PATCH] fix the racy check_hung_uninterruptible_tasks()->rcu_lock_break() logic |
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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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