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

[Greg KH <gregkh@linuxfoundation.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
> dispite of task 3 is waiting it, and this prevents the deadlock.
> It seems there is no better way to detect such the situations,
> also in general it's not good to wait so long for readers with
> interrupts disabled, since read_lock may nest with another locks
> and delay the system.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  kernel/locking/qrwlock.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>