Messages in this thread Patch in this message | | | Subject | Re: [PATCH] de_thread: Move notify_count write under lock | From | Kirill Tkhai <> | Date | Thu, 5 Feb 2015 19:11:51 +0300 |
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В Чт, 05/02/2015 в 14:38 +0100, Oleg Nesterov пишет: > On 02/05, Kirill Tkhai wrote: > > > > The write operation may be reordered with the setting of group_exit_task. > > If so, this fires in exit_notify(). > > How? > > OK, yes, "sig->notify_count = -1" can be reordered with the last unlock, > but we do not care? > > group_exit_task + notify_count is only checked under the same lock, and > "notify_count = -1" can't happen until de_thread() sees it is zero. > > Could you explain why this is bad in more details? > > > > --- a/fs/exec.c > > +++ b/fs/exec.c > > @@ -920,10 +920,16 @@ static int de_thread(struct task_struct *tsk) > > if (!thread_group_leader(tsk)) { > > struct task_struct *leader = tsk->group_leader; > > > > - sig->notify_count = -1; /* for exit_notify() */ > > for (;;) { > > threadgroup_change_begin(tsk); > > write_lock_irq(&tasklist_lock); > > + /* > > + * We could set it once outside the for() cycle, but > > + * this requires to use SMP barriers there and in > > + * exit_notify(), because the write operation may > > + * be reordered with the setting of group_exit_task. > > + */ > > + sig->notify_count = -1; /* for exit_notify() */ > > if (likely(leader->exit_state)) > > break; > > __set_current_state(TASK_KILLABLE); > > Perhaps something like this makes sense anyway to make the code more > clear, but in this case I'd suggest to set ->notify_count after we > check ->exit_state. And without the (afaics!) misleading comment... > > Or I missed something?
Other solution is in the patch below.
Can't (sig->notify_count == -1) be visible earlier than tsk->signal->group_exit_task in exit_notify()?
tasklist_lock is held in exit_notify(), but de_thread() actions (notify_count and group_exit_task writes) are independent from it (another lock is held there).
diff --git a/fs/exec.c b/fs/exec.c index ad8798e..e3235b7 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -920,6 +920,7 @@ static int de_thread(struct task_struct *tsk) if (!thread_group_leader(tsk)) { struct task_struct *leader = tsk->group_leader; + smp_wmb(); /* Pairs with smp_rmb() in exit_notify */ sig->notify_count = -1; /* for exit_notify() */ for (;;) { threadgroup_change_begin(tsk); diff --git a/kernel/exit.c b/kernel/exit.c index 6806c55..665fe0e 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -615,8 +615,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead) list_add(&tsk->ptrace_entry, &dead); /* mt-exec, de_thread() is waiting for group leader */ - if (unlikely(tsk->signal->notify_count < 0)) + if (unlikely(tsk->signal->notify_count < 0)) { + smp_rmb(); /* Pairs with smp_wmb() in de_thread */ wake_up_process(tsk->signal->group_exit_task); + } write_unlock_irq(&tasklist_lock); list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
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