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Date: Wed, 30 Sep 2020 13:04:32 +0100 From: Will Deacon <will@...nel.org> To: linux-serial@...r.kernel.org Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org, Russell King <linux@...linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, Will Deacon <will@...nel.org> Subject: [PATCH] serial: pl011: Fix lockdep splat when handling magic-sysrq interrupt From: Peter Zijlstra <peterz@...radead.org> Issuing a magic-sysrq via the PL011 causes the following lockdep splat, which is easily reproducible under QEMU: | sysrq: Changing Loglevel | sysrq: Loglevel set to 9 | | ====================================================== | WARNING: possible circular locking dependency detected | 5.9.0-rc7 #1 Not tainted | ------------------------------------------------------ | systemd-journal/138 is trying to acquire lock: | ffffab133ad950c0 (console_owner){-.-.}-{0:0}, at: console_lock_spinning_enable+0x34/0x70 | | but task is already holding lock: | ffff0001fd47b098 (&port_lock_key){-.-.}-{2:2}, at: pl011_int+0x40/0x488 | | which lock already depends on the new lock. [...] | Possible unsafe locking scenario: | | CPU0 CPU1 | ---- ---- | lock(&port_lock_key); | lock(console_owner); | lock(&port_lock_key); | lock(console_owner); | | *** DEADLOCK *** The issue being that CPU0 takes 'port_lock' on the irq path in pl011_int() before taking 'console_owner' on the printk() path, whereas CPU1 takes the two locks in the opposite order on the printk() path due to setting the "console_owner" prior to calling into into the actual console driver. Fix this in the same way as the msm-serial driver by dropping 'port_lock' before handling the sysrq. Cc: <stable@...r.kernel.org> # 4.19+ Cc: Russell King <linux@...linux.org.uk> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org> Cc: Jiri Slaby <jirislaby@...nel.org> Link: https://lore.kernel.org/r/20200811101313.GA6970@willie-the-truck Signed-off-by: Peter Zijlstra <peterz@...radead.org> Tested-by: Will Deacon <will@...nel.org> Signed-off-by: Will Deacon <will@...nel.org> --- drivers/tty/serial/amba-pl011.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 67498594d7d7..87dc3fc15694 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; -- 2.28.0.709.gb0816b6eb0-goog
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