serial: fix wakup races in the mrst_max3110 driver

The mrst_max3110 driver had a set of unsafe wakeup sequences
along the following line:

if (!atomic_read(&foo)) {
  atomic_set(&foo, 1);
  wake_up(worker_thread);
}
and the worker thread would do

if (atomic_read(&foo)) {
  do_work();
  atomic_set(&foo, 0);
}

which can result in various missed wakups due to test-then-set races,
as well as due to clear-after-work instead of clear-before-work.

This patch fixes these races by using the proper bit test-and-set operations,
and by doing clear-before-work.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/serial/mrst_max3110.c b/drivers/serial/mrst_max3110.c
index 0341853e0c2854c31797cab470387f21ea01229d..f6ad1ecbff79ec9688fbc024a2036f351efff26e 100644 (file)
--- a/drivers/serial/mrst_max3110.c
+++ b/drivers/serial/mrst_max3110.c
@@ -48,6 +48,10 @@
 
 #define PR_FMT "mrst_max3110: "
 
+#define UART_TX_NEEDED 1
+#define CON_TX_NEEDED  2
+#define BIT_IRQ_PENDING    3
+
 struct uart_max3110 {
        struct uart_port port;
        struct spi_device *spi;
@@ -63,15 +67,13 @@ struct uart_max3110 {
        u8 clock;
        u8 parity, word_7bits;
 
-       atomic_t uart_tx_need;
+       unsigned long uart_flags;
 
        /* console related */
        struct circ_buf con_xmit;
-       atomic_t con_tx_need;
 
        /* irq related */
        u16 irq;
-       atomic_t irq_pending;
 };
 
 /* global data structure, may need be removed */
@@ -176,10 +178,9 @@ static void serial_m3110_con_putchar(struct uart_port *port, int ch)
                xmit->head = (xmit->head + 1) & (PAGE_SIZE - 1);
        }
 
-       if (!atomic_read(&max->con_tx_need)) {
-               atomic_set(&max->con_tx_need, 1);
+
+       if (!test_and_set_bit(CON_TX_NEEDED, &max->uart_flags))
                wake_up_process(max->main_thread);
-       }
 }
 
 /*
@@ -318,10 +319,8 @@ static void serial_m3110_start_tx(struct uart_port *port)
        struct uart_max3110 *max =
                container_of(port, struct uart_max3110, port);
 
-       if (!atomic_read(&max->uart_tx_need)) {
-               atomic_set(&max->uart_tx_need, 1);
+       if (!test_and_set_bit(UART_TX_NEEDED, &max->uart_flags))
                wake_up_process(max->main_thread);
-       }
 }
 
 static void receive_chars(struct uart_max3110 *max, unsigned char *str, int len)
@@ -392,32 +391,23 @@ static int max3110_main_thread(void *_max)
        pr_info(PR_FMT "start main thread\n");
 
        do {
-               wait_event_interruptible(*wq, (atomic_read(&max->irq_pending) ||
-                                              atomic_read(&max->con_tx_need) ||
-                                            atomic_read(&max->uart_tx_need)) ||
-                                            kthread_should_stop());
+               wait_event_interruptible(*wq, max->uart_flags || kthread_should_stop());
 
                mutex_lock(&max->thread_mutex);
 
-#ifdef CONFIG_MRST_MAX3110_IRQ
-               if (atomic_read(&max->irq_pending)) {
+               if (test_and_clear_bit(BIT_IRQ_PENDING, &max->uart_flags))
                        max3110_console_receive(max);
-                       atomic_set(&max->irq_pending, 0);
-               }
-#endif
 
                /* first handle console output */
-               if (atomic_read(&max->con_tx_need)) {
+               if (test_and_clear_bit(CON_TX_NEEDED, &max->uart_flags))
                        send_circ_buf(max, xmit);
-                       atomic_set(&max->con_tx_need, 0);
-               }
 
                /* handle uart output */
-               if (atomic_read(&max->uart_tx_need)) {
+               if (test_and_clear_bit(UART_TX_NEEDED, &max->uart_flags))
                        transmit_char(max);
-                       atomic_set(&max->uart_tx_need, 0);
-               }
+
                mutex_unlock(&max->thread_mutex);
+
        } while (!kthread_should_stop());
 
        return ret;
@@ -430,10 +420,9 @@ static irqreturn_t serial_m3110_irq(int irq, void *dev_id)
 
        /* max3110's irq is a falling edge, not level triggered,
         * so no need to disable the irq */
-       if (!atomic_read(&max->irq_pending)) {
-               atomic_inc(&max->irq_pending);
+       if (!test_and_set_bit(BIT_IRQ_PENDING, &max->uart_flags))
                wake_up_process(max->main_thread);
-       }
+
        return IRQ_HANDLED;
 }
 #else
@@ -753,7 +742,8 @@ static int serial_m3110_probe(struct spi_device *spi)
        max->baud = 0;
 
        max->cur_conf = 0;
-       atomic_set(&max->irq_pending, 0);
+       max->uart_flags = 0;
+
        /* Check if reading configuration register returns something sane */
 
        res = RC_TAG;