On Fri, Mar 19, 2021 at 06:36:39AM +0000, Song Bao Hua (Barry Song) wrote:
>
>
> > -----Original Message-----
> > From: Andy Shevchenko [mailto:[email protected]]
> > Sent: Tuesday, March 16, 2021 10:41 PM
> > To: Johan Hovold <[email protected]>; Finn Thain
> > <[email protected]>;
> > Song Bao Hua (Barry Song) <[email protected]>
> > Cc: Krzysztof Kozlowski <[email protected]>; Greg
> > Kroah-Hartman <[email protected]>; Jiri Slaby
> > <[email protected]>;
> > linux-arm Mailing List <[email protected]>; Linux Samsung
> > SOC <[email protected]>; open list:SERIAL DRIVERS
> > <[email protected]>; Linux Kernel Mailing List
> > <[email protected]>; Hector Martin <[email protected]>; Arnd
> > Bergmann <[email protected]>
> > Subject: Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in
> > interrupt handlers
> >
> > On Tue, Mar 16, 2021 at 11:02 AM Johan Hovold <[email protected]> wrote:
> > >
> > > On Mon, Mar 15, 2021 at 07:12:12PM +0100, Krzysztof Kozlowski wrote:
> > > > Since interrupt handler is called with disabled local interrupts, there
> > > > is no need to use the spinlock primitives disabling interrupts as well.
> > >
> > > This isn't generally true due to "threadirqs" and that can lead to
> > > deadlocks if the console code is called from hard irq context.
> > >
> > > Now, this is *not* the case for this particular driver since it doesn't
> > > even bother to take the port lock in console_write(). That should
> > > probably be fixed instead.
> > >
> > > See https://lore.kernel.org/r/X7kviiRwuxvPxC8O@localhost.
> >
> > Finn, Barry, something to check I think?
>
> My understanding is that spin_lock_irqsave can't protect the context
> the console_write() is called in hardirq for threaded_irq case mainly
> for preempt-rt scenarios as spin_lock_irqsave doesn't disable irq in
> that case at all.
Forced threaded interrupts have so far run with interrupts enabled and
spin_lock_irqsave() would suffice on non-RT. This is about to change
though so that drivers don't need to worry about "threadirqs":
https://lore.kernel.org/r/[email protected]
> See:
> https://www.kernel.org/doc/html/latest/locking/locktypes.html
> spinlock_t and PREEMPT_RT
> On a PREEMPT_RT kernel spinlock_t is mapped to a separate implementation
> based on rt_mutex which changes the semantics:
> Preemption is not disabled.
> The hard interrupt related suffixes for spin_lock / spin_unlock operations
> (_irq, _irqsave / _irqrestore) do not affect the CPU’s interrupt disabled
> state.
>
> So if console_write() can interrupt our code in hardirq, we should
> move to raw_spin_lock_irqsave for this driver.
No, no. RT handles this by deferring console writes apparently.
> I think it is almost always wrong to call spin_lock_irqsave in hardirq.
Again, no. It's even been a requirement due to "threadirqs" in some
cases (e.g. hrtimers) up until now (or rather until the above patch is
in mainline).
Johan
