On 04.04.2018 18:35, Peter Zijlstra wrote: > On Wed, Apr 04, 2018 at 06:24:39PM +0300, Kirill Tkhai wrote: >> The following situation leads to deadlock: >> >> [task 1] [task 2] [task 3] >> kill_fasync() mm_update_next_owner() >> copy_process() >> spin_lock_irqsave(&fa->fa_lock) read_lock(&tasklist_lock) >> write_lock_irq(&tasklist_lock) >> send_sigio() <IRQ> ... >> read_lock(&fown->lock) kill_fasync() ... >> read_lock(&tasklist_lock) spin_lock_irqsave(&fa->fa_lock) ... >> >> Task 1 can't acquire read locked tasklist_lock, since there is >> already task 3 expressed its wish to take the lock exclusive. >> Task 2 holds the read locked lock, but it can't take the spin lock. >> >> The patch makes queued_read_lock_slowpath() to give task 1 the same >> priority as it was an interrupt handler, and to take the lock > > That re-introduces starvation scenarios. And the above looks like a > proper deadlock that should be sorted by fixing the locking order.
We can move tasklist_lock out of send_sigio(), but I'm not sure it's possible for read_lock(&fown->lock). Is there another solution? Is there reliable way to iterate do_each_pid_task() with rcu_read_lock()?