On 04/04/2018 11:55 AM, Kirill Tkhai wrote: > 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. One possible solution is add a flag in send_sigio() to use a read_trylock(&tasklist_lock) instead of read_lock(). If the trylock fails, returns an error and have the caller (kill_fasync) release fa->fa_lock and retry again. Task 1 has 3 levels of nested locking and so it should be the one that does a retry if the innermost locking fails. An warning can be printed if the retry count is too large. -Longman