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);
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;
}
--
2.25.1
