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SubjectRe: possible deadlock in __ata_sff_interrupt
On 12/16/22 22:05, Al Viro wrote:
> 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 <> 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 ( 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?
That may not be a good idea as the cacheline bouncing problem will be
back with reduced performance.
> ISTR threads about the possibility of explicit read_lock_unfair()...

Another possible alternative is to treat the read_lock as unfair if
interrupt has been disabled as I think we should reduce the interrupt
disabled interval as much as possible.



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