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

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----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.
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.

I think it is almost always wrong to call spin_lock_irqsave in hardirq.

> 
> --
> With Best Regards,
> Andy Shevchenko

Thanks
Barry