Re: possible deadlock in __ata_sff_interrupt

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Dec 16, 2022 at 08:31:54PM -0600, Linus Torvalds wrote:
> Ok, let's bring in Waiman for the rwlock side.
> 
> On Fri, Dec 16, 2022 at 5:54 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > Right, for a reader not in_interrupt(), it may be blocked by a random
> > waiting writer because of the fairness, even the lock is currently held
> > by a reader:
> >
> >         CPU 1                   CPU 2           CPU 3
> >         read_lock(&tasklist_lock); // get the lock
> >
> >                                                 write_lock_irq(&tasklist_lock); // wait for the lock
> >
> >                                 read_lock(&tasklist_lock); // cannot get the lock because of the fairness
> 
> But this should be ok - because CPU1 can make progress and eventually
> release the lock.
> 
> So the tasklist_lock use is fine on its own - the reason interrupts
> are special is because an interrupt on CPU 1 taking the lock for
> reading would deadlock otherwise. As long as it happens on another
> CPU, the original CPU should then be able to make progress.
> 
> But the problem here seems to be thst *another* lock is also involved
> (in this case apparently "host->lock", and now if CPU1 and CPU2 get
> these two locks in a different order, you can get an ABBA deadlock.
> 
> And apparently our lockdep machinery doesn't catch that issue, so it
> doesn't get flagged.

Lockdep has actually caught that; the locks involved are mention in the
report (https://marc.info/?l=linux-ide&m=167094379710177&w=2).  The form
of report might have been better, but if anything, it doesn't mention
potential involvement of tasklist_lock writer, turning that into a deadlock.

OTOH, that's more or less implicit for the entire class:

read_lock(A)		[non-interrupt]
			local_irq_disable()	local_irq_disable()
			spin_lock(B)		write_lock(A)
			read_lock(A)
[in interrupt]
spin_lock(B)

is what that sort of reports is about.  In this case A is tasklist_lock,
B is host->lock.  Possible call chains for CPU1 and CPU2 are reported...

I wonder why analogues of that hadn't been reported for other SCSI hosts -
it's a really common pattern there...

> I'm not sure what the lockdep rules for rwlocks are, but maybe lockdep
> treats rwlocks as being _always_ unfair, not knowing about that "it's
> only unfair when it's in interrupt context".
> 
> Maybe we need to always make rwlock unfair? Possibly only for tasklist_lock?

ISTR threads about the possibility of explicit read_lock_unfair()...