From: Will Deacon <will@kernel.org>
To: peterz@infradead.org
Cc: gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
linux@armlinux.org.uk, linux-arm-kernel@lists.infradead.org,
andre.przywara@arm.com
Subject: Re: lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8
Date: Tue, 11 Aug 2020 12:17:13 +0100 [thread overview]
Message-ID: <20200811111713.GA7203@willie-the-truck> (raw)
In-Reply-To: <20200811103841.GC2674@hirez.programming.kicks-ass.net>
On Tue, Aug 11, 2020 at 12:38:41PM +0200, peterz@infradead.org wrote:
> On Tue, Aug 11, 2020 at 11:13:13AM +0100, Will Deacon wrote:
> > Using magic-sysrq via a keyboard interrupt over the serial console results in
> > the following lockdep splat with the PL011 UART driver on v5.8. I can reproduce
> > the issue under QEMU with arm64 defconfig + PROVE_LOCKING.
> >
> > Any chance somebody could take a look, please? It's a little annoying,
> > because it means when I uses magic-sysrq to increase the loglevel prior
> > to testing something else, lockdep gets disabled as a result.
> >
>
> Going by msm_serial, the thing to do is something like this:
>
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8efd7c2a34fe..1717790ece2b 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -308,8 +308,9 @@ static void pl011_write(unsigned int val, const struct uart_amba_port *uap,
> */
> static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> {
> - u16 status;
> unsigned int ch, flag, fifotaken;
> + int sysrq;
> + u16 status;
>
> for (fifotaken = 0; fifotaken != 256; fifotaken++) {
> status = pl011_read(uap, REG_FR);
> @@ -344,10 +345,12 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> flag = TTY_FRAME;
> }
>
> - if (uart_handle_sysrq_char(&uap->port, ch & 255))
> - continue;
> + spin_unlock(&uap->port.lock);
> + sysrq = uart_handle_sysrq_char(&uap->port, ch & 255);
> + spin_lock(&uap->port.lock);
>
> - uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> + if (!sysrq)
> + uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> }
>
> return fifotaken;
Cheers, that seems to do the trick:
Tested-by: Will Deacon <will@kernel.org>
but what I don't understand is why I haven't run into this before, and why
nobody else seems to be reporting it!
I'll try some older kernels to see if it ever worked.
Will
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next prev parent reply other threads:[~2020-08-11 11:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-11 10:13 lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8 Will Deacon
2020-08-11 10:38 ` peterz
2020-08-11 11:17 ` Will Deacon [this message]
2020-08-11 12:36 ` Will Deacon
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