Messages in this thread | | | Date | Tue, 11 Aug 2020 12:17:13 +0100 | From | Will Deacon <> | Subject | Re: lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8 |
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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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