[PATCH v2] tty: serial: imx: fix potential deadlock

[Sam Nobs <samuel.nobs@taitradio.com>]

Enabling the lock dependency validator has revealed
that the way spinlocks are used in the IMX serial
port could result in a deadlock.

Specifically, imx_uart_int() acquires a spinlock
without disabling the interrupts, meaning that another
interrupt could come along and try to acquire the same
spinlock, potentially causing the two to wait for each
other indefinitely.

Use spin_lock_irqsave() instead to disable interrupts
upon acquisition of the spinlock.

Signed-off-by: Sam Nobs <samuel.nobs@taitradio.com>
---
patch changelog:
 v2 - added comment explaining why we need the irqsave variant of spin_lock(),
      as suggested by Uwe

Here's the lockdep splat for reference:

   ================================
   WARNING: inconsistent lock state
   5.4.46 #1 Not tainted
   --------------------------------
   inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
   irq/26-30860000/992 [HC0[0]:SC0[2]:HE1:SE0] takes:
   ffff00006671d098 (&port_lock_key){?...}, at: imx_uart_int+0x28/0x338
   {IN-HARDIRQ-W} state was registered at:
     lock_acquire+0xd0/0x288
     _raw_spin_lock_irqsave+0x58/0x80
     imx_uart_console_write+0x1e4/0x220
     console_unlock+0x2ac/0x610
     vprintk_emit+0x178/0x330
     vprintk_default+0x48/0x58
     vprintk_func+0xe4/0x248
     printk+0x70/0x90
     crng_fast_load+0x1c4/0x1c8
     add_interrupt_randomness+0x348/0x3e8
     handle_irq_event_percpu+0x50/0x98
     handle_irq_event+0x4c/0xd0
     handle_fasteoi_irq+0xe0/0x188
     generic_handle_irq+0x34/0x50
     __handle_domain_irq+0x98/0x108
     gic_handle_irq+0xd4/0x178
     el1_irq+0xbc/0x180
     arch_cpu_idle+0x34/0x220
     default_idle_call+0x40/0x4c
     do_idle+0x254/0x268
     cpu_startup_entry+0x28/0x48
     rest_init+0x1b4/0x284
     arch_call_rest_init+0x14/0x1c
     start_kernel+0x48c/0x4bc
   irq event stamp: 30
   hardirqs last  enabled at (29): [<ffff800010b22a60>] _raw_spin_unlock_irq+0x38/0x70
   hardirqs last disabled at (28): [<ffff800010b1b060>] __schedule+0xb0/0x770
   softirqs last  enabled at (0): [<ffff8000100b28c0>] copy_process+0x8d8/0x19b0
   softirqs last disabled at (30): [<ffff8000101343f8>] irq_forced_thread_fn+0x0/0xc0

   other info that might help us debug this:
    Possible unsafe locking scenario:

          CPU0
          ----
     lock(&port_lock_key);
     <Interrupt>
       lock(&port_lock_key);

    *** DEADLOCK ***

   no locks held by irq/26-30860000/992.

   stack backtrace:
   CPU: 0 PID: 992 Comm: irq/26-30860000 Not tainted 5.4.46 #1
   Hardware name: Tait i.MX8MM smarc-rcb (DT)
   Call trace:
    dump_backtrace+0x0/0x178
    show_stack+0x24/0x30
    dump_stack+0xdc/0x144
    print_usage_bug+0x1c8/0x1e0
    mark_lock+0x57c/0x740
    __lock_acquire+0x684/0x16d0
    lock_acquire+0xd0/0x288
    _raw_spin_lock+0x44/0x58
    imx_uart_int+0x28/0x338
    irq_forced_thread_fn+0x40/0xc0
    irq_thread+0x1ac/0x280
    kthread+0x138/0x140
    ret_from_fork+0x10/0x18


 drivers/tty/serial/imx.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 1731d97..2185184 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -942,8 +942,14 @@ static irqreturn_t imx_uart_int(int irq, void *dev_id)
 	struct imx_port *sport = dev_id;
 	unsigned int usr1, usr2, ucr1, ucr2, ucr3, ucr4;
 	irqreturn_t ret = IRQ_NONE;
-
-	spin_lock(&sport->port.lock);
+	unsigned long flags = 0;
+
+	/*
+	 * IRQs might not be disabled upon entering this interrupt handler, 
+	 * e.g. when interrupt handlers are forced to be threaded. To support
+	 * this scenario as well, disable IRQs when acquiring the spinlock.
+	 */
+	spin_lock_irqsave(&sport->port.lock, flags);
 
 	usr1 = imx_uart_readl(sport, USR1);
 	usr2 = imx_uart_readl(sport, USR2);
@@ -1013,7 +1019,7 @@ static irqreturn_t imx_uart_int(int irq, void *dev_id)
 		ret = IRQ_HANDLED;
 	}
 
-	spin_unlock(&sport->port.lock);
+	spin_unlock_irqrestore(&sport->port.lock, flags);
 
 	return ret;
 }
-- 
2.7.4