Messages in this thread | | | Date | Sat, 3 May 2014 18:11:33 +0200 | From | Oleg Nesterov <> | Subject | lock_task_sighand() && rcu_boost() |
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Paul,
I just noticed by accident that __lock_task_sighand() looks ugly and mysterious ;) And I am puzzled.
a841796f11c90d53 "signal: align __lock_task_sighand() irq disabling and RCU" says:
The __lock_task_sighand() function calls rcu_read_lock() with interrupts and preemption enabled, but later calls rcu_read_unlock() with interrupts disabled. It is therefore possible that this RCU read-side critical section will be preempted and later RCU priority boosted, which means that rcu_read_unlock() will call rt_mutex_unlock() in order to deboost itself, but with interrupts disabled. This results in lockdep splats ...
OK, if we can't rcu_read_unlock() with irqs disabled, then we can at least cleanup it (and document the problem). Say,
struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, unsigned long *flags) { struct sighand_struct *sighand;
rcu_read_lock(); for (;;) { sighand = rcu_dereference(tsk->sighand); if (unlikely(sighand == NULL)) break;
spin_lock_irqsave(&sighand->siglock, *flags); /* * We delay rcu_read_unlock() till unlock_task_sighand() * to avoid rt_mutex_unlock(current->rcu_boost_mutex) with * irqs disabled. */ if (likely(sighand == tsk->sighand)) return sighand; spin_unlock_irqrestore(&sighand->siglock, *flags); } rcu_read_unlock();
return sighand; /* NULL */ }
and add rcu_read_unlock() into unlock_task_sighand().
But. I simply can't understand why lockdep should complain? Why it is bad to lock/unlock ->wait_lock with irqs disabled?
wakeup_next_waiter() and rt_mutex_adjust_prio() should be fine, they start with _irqsave().
The changelog also says:
It is quite possible that a better long-term fix is to make rt_mutex_unlock() disable irqs when acquiring the rt_mutex structure's ->wait_lock.
and if it is actually bad, then how the change above can fix the problem?
Help!
Oleg.
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