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

[Kirill Tkhai]

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()?