Re: [PATCH] locking/qrwlock: Give priority to readers with irqs disabled to prevent deadlock

[Waiman Long <longman@redhat.com>]

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