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From: Thomas Gleixner <tglx@linutronix.de>
To: Leonardo Bras <leobras@redhat.com>, Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>, Jiri Slaby <jirislaby@kernel.org>, Ilpo
 =?utf-8?Q?J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>, Andy Shevchenko
 <andriy.shevchenko@linux.intel.com>, Florian Fainelli
 <f.fainelli@gmail.com>, John Ogness <john.ogness@linutronix.de>, Tony
 Lindgren <tony@atomide.com>, Marcelo Tosatti <mtosatti@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Sebastian
 Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [RESEND RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT
 atomic context
In-Reply-To: <20240116073701.2356171-3-leobras@redhat.com>
References: <20240116073701.2356171-1-leobras@redhat.com>
 <20240116073701.2356171-3-leobras@redhat.com>
Date: Wed, 17 Jan 2024 23:44:55 +0100
Message-ID: <87r0ifful4.ffs@tglx>
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On Tue, Jan 16 2024 at 04:37, Leonardo Bras wrote:
> With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
> spin_lock(), without preempt_disable() or irq_disable().
>
> This allows a task T1 to get preempted or interrupted while holding the
> port->lock. If the preempting task T2 need the lock, spin_lock() code
> will schedule T1 back until it finishes using the lock, and then go back to
> T2.
>
> There is an issue if a T1 holding port->lock is interrupted by an
> IRQ, and this IRQ handler needs to get port->lock for writting (printk):
> spin_lock() code will try to reschedule the interrupt handler, which is in
> atomic context, causing a BUG() for trying to reschedule/sleep in atomic
> context.
>
> So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
> fails proceed anyway, just like it's done in oops_in_progress case.

That's just blantantly wrong. The locks are really only to be ignored
for the oops case, but not for regular printk.

I assume that this is not against the latest RT kernel as that should
not have that problem at all.

Thanks,

        tglx