Messages in this thread Patch in this message | | | From | Andrey Smirnov <> | Subject | [RFC, PATCH] imx: serial: Take tty->files_lock opportunistically | Date | Tue, 30 May 2017 05:37:26 -0700 |
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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: cphealy@gmail.com Cc: Peter Senna Tschudin <peter.senna@collabora.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Jiri Slaby <jslaby@suse.com> Cc: linux-kernel@vger.kernel.org Signed-off-by: Andrey Smirnov <andrew.smirnov@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
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