On Wed, Sep 29, 2021 at 06:43:24PM -0300, Fabio Estevam wrote:
> The following sysrq command causes the following deadlock:
>
> # echo t > /proc/sysrq-trigger
> ....
> [ 20.325246] ======================================================
> [ 20.325252] WARNING: possible circular locking dependency detected
> [ 20.325260] 5.15.0-rc2-next-20210924-00004-gd2d6e664f29f-dirty #163
> Not tainted
> [ 20.325273] ------------------------------------------------------
> [ 20.325279] sh/236 is trying to acquire lock:
> [ 20.325293] c1618614 (console_owner){-...}-{0:0}, at:
> console_unlock+0x180/0x5bc
> [ 20.325361]
> [ 20.325361] but task is already holding lock:
> [ 20.325368] eefccc90 (&pool->lock){-.-.}-{2:2}, at:
> show_workqueue_state+0x104/0x3c8
> [ 20.325432]
> [ 20.325432] which lock already depends on the new lock.
>
> ...
>
> [ 20.325657] -> #2 (&pool->lock/1){-.-.}-{2:2}:
> [ 20.325690] __queue_work+0x114/0x810
> [ 20.325710] queue_work_on+0x54/0x94
> [ 20.325727] __imx_uart_rxint.constprop.0+0x1b4/0x2e0
> [ 20.325760] imx_uart_int+0x270/0x310
>
> This problem happens because uart_handle_sysrq_char() is called
> with the lock held.
>
> Fix this by using the same approach done in commit 5697df7322fe ("serial:
> fsl_lpuart: split sysrq handling"), which calls
> uart_unlock_and_check_sysrq() to drop the lock prior to
> uart_handle_sysrq_char().
>
> Signed-off-by: Fabio Estevam <festevam@xxxxxxx>
> ---
> Changes since v1:
> - I noticed that when sending break + t via the terminal, the characters
> were sometimes lost. Do the minimal changes to fix the deadlock without
> missing the sysrq input.
>
> drivers/tty/serial/imx.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 8b121cd869e9..1c768dd3896d 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -788,6 +788,7 @@ static irqreturn_t __imx_uart_rxint(int irq, void *dev_id)
> unsigned int rx, flg, ignored = 0;
> struct tty_port *port = &sport->port.state->port;
>
> + uart_unlock_and_check_sysrq(&sport->port);
This is just so broken; you can't just drop the lock. And you clearly
haven't even tried to understand how uart_unlock_and_check_sysrq()
works.
Please take a closer look at the commit you're trying to mimic.
> while (imx_uart_readl(sport, USR2) & USR2_RDR) {
> u32 usr2;
>
> @@ -846,6 +847,7 @@ static irqreturn_t __imx_uart_rxint(int irq, void *dev_id)
> out:
> tty_flip_buffer_push(port);
>
> + spin_lock(&sport->port.lock);
> return IRQ_HANDLED;
> }
Johan
