Trying to load a serdev driver agains a tty port on i.MX6Q results in
the following lockdep warning:
kworker/u8:1/100 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
(&(&tty->files_lock)->rlock){+.+...}, at: [<c04d3d4c>] imx_startup+0x2c0/0x50c
and this task is already holding:
(&port_lock_key){-.....}, at: [<c04d3b98>] imx_startup+0x10c/0x50c
which would create a new lock dependency:
(&port_lock_key){-.....} -> (&(&tty->files_lock)->rlock){+.+...}
but this new dependency connects a HARDIRQ-irq-safe lock:
(&port_lock_key){-.....}
... which became HARDIRQ-irq-safe at:
lock_acquire+0x74/0x94
_raw_spin_lock_irqsave+0x40/0x54
imx_txint+0x18/0x1d8
imx_int+0xc0/0x21c
__handle_irq_event_percpu+0x8c/0x124
handle_irq_event_percpu+0x24/0x60
handle_irq_event+0x40/0x64
handle_fasteoi_irq+0xd4/0x1ac
generic_handle_irq+0x28/0x3c
__handle_domain_irq+0x6c/0xe8
gic_handle_irq+0x58/0xb8
__irq_svc+0x70/0x98
cpuidle_enter_state+0x18c/0x2b8
cpuidle_enter+0x1c/0x20
call_cpuidle+0x28/0x44
do_idle+0x1b0/0x224
cpu_startup_entry+0x20/0x24
rest_init+0x128/0x168
start_kernel+0x328/0x39c
0x1000807c
to a HARDIRQ-irq-unsafe lock:
(&(&tty->files_lock)->rlock){+.+...}
... which became HARDIRQ-irq-unsafe at:
...
lock_acquire+0x74/0x94
_raw_spin_lock+0x30/0x40
tty_add_file+0x28/0x50
tty_open+0x9c/0x490
chrdev_open+0xa4/0x180
do_dentry_open+0x1f0/0x318
vfs_open+0x54/0x84
path_openat+0x32c/0xfbc
do_filp_open+0x68/0xcc
do_sys_open+0x108/0x1d0
SyS_open+0x20/0x24
kernel_init_freeable+0x15c/0x200
kernel_init+0x10/0x11c
ret_from_fork+0x14/0x24
other info that might help us debug this:
Possible interrupt unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&(&tty->files_lock)->rlock);
local_irq_disable();
lock(&port_lock_key);
lock(&(&tty->files_lock)->rlock);
<Interrupt>
lock(&port_lock_key);
*** DEADLOCK ***
In order to avoid this problem, change the code to opportunisticaly
take 'tty->files_lock' by using spin_trylock() in the offending
codepath instead.
Fixes: 18a4208826dd0a13eb06de724c86bba2c225f943 ("imx-serial: Reduce
RX DMA startup latency when opening for reading")
Cc: cphe...@gmail.com
Cc: Peter Senna Tschudin <peter.se...@collabora.com>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Cc: Jiri Slaby <jsl...@suse.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrey Smirnov <andrew.smir...@gmail.com>
---
Not sure if this is the best way to solve the problem (hence the RFC
tag). If anyone has a better idea, or if there's a better fix for this
already, please let me know.
Thanks,
Andrey Smirnov
drivers/tty/serial/imx.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 33509b4..24fe7fc 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -1350,14 +1350,14 @@ static int imx_startup(struct uart_port *port)
struct tty_file_private *file_priv;
int readcnt = 0;
- spin_lock(&tty->files_lock);
+ if (spin_trylock(&tty->files_lock)) {
+ if (!list_empty(&tty->tty_files))
+ list_for_each_entry(file_priv, &tty->tty_files,
list)
+ if (!(file_priv->file->f_flags &
O_WRONLY))
+ readcnt++;
- if (!list_empty(&tty->tty_files))
- list_for_each_entry(file_priv, &tty->tty_files, list)
- if (!(file_priv->file->f_flags & O_WRONLY))
- readcnt++;
-
- spin_unlock(&tty->files_lock);
+ spin_unlock(&tty->files_lock);
+ }
if (readcnt > 0) {
imx_disable_rx_int(sport);
--
2.9.4
