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SubjectRe: [PATCH] locking/qrwlock: Give priority to readers with irqs disabled to prevent deadlock
On Wed, Apr 04, 2018 at 06:51:08PM +0300, 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).

So the scenario is:

CPU0 CPU1 CPU2

spin_lock_irqsave(&fa->fa_lock);
read_lock(&tasklist_lock)
write_lock_irq(&tasklist_lock)
read_lock(&tasklist_lock)
<IRQ>
spin_lock_irqsave(&fa->fa_lock);


Right? (where the row now signifies time)

That doesn't seem to include fown->lock, you're saying it has an
identical issue?

> Is there another solution? Is there reliable way to iterate do_each_pid_task()
> with rcu_read_lock()?

Depends on what you call reliable :-), Yes you can use
do_each_pid_task() with RCU, but as always you're prone to see tasks
that are dead and miss tasks that just came in.

If that is sufficient for the signal muck, dunno :/ Typically signals
use sighand lock, not tasklist_lock.


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