Re: possible deadlock in __ata_sff_interrupt

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

[Boqun Feng Cc'd]

On Fri, Dec 16, 2022 at 03:26:21AM -0800, Linus Torvalds wrote:
> On Thu, Dec 15, 2022 at 7:41 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > CPU1: ptrace(2)
> >         ptrace_check_attach()
> >                 read_lock(&tasklist_lock);
> >
> > CPU2: setpgid(2)
> >         write_lock_irq(&tasklist_lock);
> >         spins
> >
> > CPU1: takes an interrupt that would call kill_fasync().  grep and the
> > first instance of kill_fasync() is in hpet_interrupt() - it's not
> > something exotic.  IRQs disabled on CPU2 won't stop it.
> >         kill_fasync(..., SIGIO, ...)
> >                 kill_fasync_rcu()
> >                         read_lock_irqsave(&fa->fa_lock, flags);
> >                         send_sigio()
> >                                 read_lock_irqsave(&fown->lock, flags);
> >                                 read_lock(&tasklist_lock);
> >
> > ... and CPU1 spins as well.
> 
> Nope. See kernel/locking/qrwlock.c:

[snip rwlocks are inherently unfair, queued ones are somewhat milder, but
all implementations have writers-starving behaviour for read_lock() at least
when in_interrupt()]

D'oh...  Consider requested "Al, you are a moron" duly delivered...  I plead
having been on way too low caffeine and too little sleep ;-/

Looking at the original report, looks like the scenario there is meant to be
the following:

CPU1: read_lock(&tasklist_lock)
	tasklist_lock grabbed

CPU2: get an sg write(2) feeding request to libata; host->lock is taken,
	request is immediately completed and scsi_done() is about to be called.
	host->lock grabbed

CPU3: write_lock_irq(&tasklist_lock)
	spins on tasklist_lock until CPU1 gets through.

CPU2: get around to kill_fasync() called by sg_rq_end_io() and to grabbing
	tasklist_lock inside send_sigio()
	spins, since it's not in an interrupt and there's a pending writer
	host->lock is held, spin until CPU3 gets through.

CPU1: take an interrupt, which on libata will try to grab host->lock
	tasklist_lock is held, spins on host->lock until CPU2 gets through

Am I reading it correctly?