Messages in this thread | | | Date | Wed, 2 Dec 2009 19:34:46 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC,PATCH 14/14] utrace core |
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On 12/01, Peter Zijlstra wrote: > > > +static inline __must_check int utrace_control_pid( > > + struct pid *pid, struct utrace_engine *engine, > > + enum utrace_resume_action action) > > +{ > > + /* > > + * We don't bother with rcu_read_lock() here to protect the > > + * task_struct pointer, because utrace_control will return > > + * -ESRCH without looking at that pointer if the engine is > > + * already detached. A task_struct pointer can't die before > > + * all the engines are detached in release_task() first. > > + */ > > + struct task_struct *task = pid_task(pid, PIDTYPE_PID); > > + return unlikely(!task) ? -ESRCH : utrace_control(task, engine, action); > > +} > > Is that comment correct? Without rcu_read_lock() the pidhash can change > under our feet and maybe cause funny things?
I already tried to answer, but I guess my email was not very clear. Let me try again.
pid_task() by itself is safe, but yes, it is possible that utrace_control() is called with target == NULL, or this task_task was already freed/reused.
utrace_control(target) path does not use target until it verifies it is safe to dereference it.
get_utrace_lock() calls rcu_read_lock() and checks that engine->ops is not cleared (NULL or utrace_detached_ops). If we see the valid ->ops under rcu_read_lock() it is safe to dereference target, even if we race with release_task() we know that it has not passed utrace_release_task() yet, and thus we know call_rcu(delayed_put_task_struct) was not yet called _before_ we took rcu_read_lock().
If it is safe to dereference target, we can take utrace->lock. Once we take this lock (and re-check engine->ops) the task can't go away until we drop it, get_utrace_lock() drops rcu lock and returns with utrace->lock held.
utrace_control() can safely play with target under utrace->lock.
> > + /* > > + * If this flag is still set it's because there was a signal > > + * handler setup done but no report_signal following it. Clear > > + * the flag before we get to user so it doesn't confuse us later. > > + */ > > + if (unlikely(utrace->signal_handler)) { > > + spin_lock(&utrace->lock); > > + utrace->signal_handler = 0; > > + spin_unlock(&utrace->lock); > > + } > > OK, so maybe you get to explain why this works..
Missed this part yesterday.
Well. ->signal_handler is set by handle_signal() when the signal was delivered to the tracee. This flag is checked by utrace_get_signal() to detect the stepping. But we should not return to user-mode with this flag set, that is why utrace_resume() clears it.
However. This reminds me we were going to try to simplify this logic, I'll try to think about this.
Oleg.
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