linux-kernel - Re: [PATCH v2] serial/pmac_zilog: Remove flawed mitigation for rx irq flood

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Message-ID: <87v84w7be3.fsf@mail.lhotse>
Date: Fri, 05 Apr 2024 14:10:12 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Finn Thain <fthain@...ux-m68k.org>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>
Cc: Nicholas Piggin <npiggin@...il.com>, Christophe Leroy
 <christophe.leroy@...roup.eu>, "Aneesh Kumar K.V"
 <aneesh.kumar@...nel.org>, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-m68k@...ts.linux-m68k.org, Benjamin
 Herrenschmidt <benh@...nel.crashing.org>, Andy Shevchenko
 <andy.shevchenko@...il.com>
Subject: Re: [PATCH v2] serial/pmac_zilog: Remove flawed mitigation for rx
 irq flood

Finn Thain <fthain@...ux-m68k.org> writes:
> The mitigation was intended to stop the irq completely. That may be
> better than a hard lock-up but it turns out that you get a crash anyway
> if you're using pmac_zilog as a serial console:
>
> ttyPZ0: pmz: rx irq flood !
> BUG: spinlock recursion on CPU#0, swapper/0
>
> That's because the pr_err() call in pmz_receive_chars() results in
> pmz_console_write() attempting to lock a spinlock already locked in
> pmz_interrupt(). With CONFIG_DEBUG_SPINLOCK=y, this produces a fatal
> BUG splat. The spinlock in question is the one in struct uart_port.
>
> Even when it's not fatal, the serial port rx function ceases to work.
> Also, the iteration limit doesn't play nicely with QEMU, as can be
> seen in the bug report linked below.
>
> A web search for other reports of the error message "pmz: rx irq flood"
> didn't produce anything. So I don't think this code is needed any more.
> Remove it.

Yeah I think you're probably right.

I assume you have tested this on an actual pmac, as well as qemu?

cheers