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

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