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

[Kirill Tkhai]

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.