Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in interrupt handlers

[Johan Hovold <johan@kernel.org>]

On Fri, Mar 19, 2021 at 06:36:39AM +0000, Song Bao Hua (Barry Song) wrote:
> 
> 
> > -----Original Message-----
> > From: Andy Shevchenko [mailto:andy.shevchenko@gmail.com]
> > Sent: Tuesday, March 16, 2021 10:41 PM
> > To: Johan Hovold <johan@kernel.org>; Finn Thain <fthain@telegraphics.com.au>;
> > Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>; Greg
> > Kroah-Hartman <gregkh@linuxfoundation.org>; Jiri Slaby <jirislaby@kernel.org>;
> > linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>; Linux Samsung
> > SOC <linux-samsung-soc@vger.kernel.org>; open list:SERIAL DRIVERS
> > <linux-serial@vger.kernel.org>; Linux Kernel Mailing List
> > <linux-kernel@vger.kernel.org>; Hector Martin <marcan@marcan.st>; Arnd
> > Bergmann <arnd@kernel.org>
> > Subject: Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in
> > interrupt handlers
> > 
> > On Tue, Mar 16, 2021 at 11:02 AM Johan Hovold <johan@kernel.org> 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/20210317143859.513307808@linutronix.de

> 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