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

[Peter Zijlstra <peterz@infradead.org>]

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.