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 13:36:58 +0100 [thread overview]
Message-ID: <20200811123657.GA7252@willie-the-truck> (raw)
In-Reply-To: <20200811111713.GA7203@willie-the-truck>
On Tue, Aug 11, 2020 at 12:17:13PM +0100, Will Deacon wrote:
> On Tue, Aug 11, 2020 at 12:38:41PM +0200, peterz@infradead.org wrote:
> > 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.
I tried 5.4, 4.19, 4.14 and 4.9:
5.4 and 4.19 have the lockdep splat
4.14 doesn't have the splat, yet I don't see why (the driver code hasn't really
changed)
4.9 magic sysrq doesn't work at all (no splat either though)
Will
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prev parent reply other threads:[~2020-08-11 12:38 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
2020-08-11 12:36 ` Will Deacon [this message]
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