On 04.04.2018 18:51, Kirill Tkhai wrote: > 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()?
In case of &fown->lock we may always disable irqs for all the places, where it's taken for read, i.e. read_lock_irqsave(&fown->lock). This seems to fix the problem for this lock.