sergiotarxz/nuttx
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serial.c: Fix a race condition noted by Stefan Kolb. Between the test if the TX buffer is full and entering a critical section, bytes may be removed from the TX buffer making the wait unnecessary. The unnecessary wait is an inefficiency, but not really a problem. But with USB CDC/ACM it can be a problem because the entire TX buffer may be emptied when we lose the race. If that happens that uart_putxmitchar() can hang waiting for data to be removed from an empty TX buffer.

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This commit is contained in:
Gregory Nutt 2016-07-22 07:47:59 -06:00
parent 369c942605
commit 9a8c3572db
1 changed files with 38 additions and 13 deletions

51
drivers/serial/serial.c

@ -208,50 +208,75 @@ static int uart_putxmitchar(FAR uart_dev_t *dev, int ch, bool oktoblock)
nexthead = 0;
}
/* Loop until we are able to add the character to the TX buffer */
/* Loop until we are able to add the character to the TX buffer. */
for (; ; )
{
/* Check if the TX buffer is full */
if (nexthead != dev->xmit.tail)
{
/* No.. not full. Add the character to the TX buffer and return. */
dev->xmit.buffer[dev->xmit.head] = ch;
dev->xmit.head = nexthead;
return OK;
}
/* The buffer is full and no data is available now. Should be block,
* waiting for the hardware to remove some data from the TX
* buffer?
/* The TX buffer is full. Should be block, waiting for the hardware
* to remove some data from the TX buffer?
*/
else if (oktoblock)
{
/* Inform the interrupt level logic that we are waiting. This and
* the following steps must be atomic.
/* The following steps must be atomic with respect to serial
* interrupt handling.
*/
flags = enter_critical_section();
/* Check again... In certain race conditions an interrupt may
* have occurred between the test at the top of the loop and
* entering the critical section and the TX buffer may no longer
* be full.
*
* NOTE: On certain devices, such as USB CDC/ACM, the entire TX
* buffer may have been emptied in this race condition. In that
* case, the logic would hang below waiting for space in the TX
* buffer without this test.
*/
if (nexthead != dev->xmit.tail)
{
ret = OK;
}
#ifdef CONFIG_SERIAL_REMOVABLE
/* Check if the removable device is no longer connected while we
* have interrupts off. We do not want the transition to occur
* as a race condition before we begin the wait.
*/
if (dev->disconnected)
else if (dev->disconnected)
{
ret = -ENOTCONN;
}
else
#endif
else
{
/* Wait for some characters to be sent from the buffer with
* the TX interrupt enabled. When the TX interrupt is
* enabled, uart_xmitchars should execute and remove some
* of the data from the TX buffer.
*/
/* Inform the interrupt level logic that we are waiting. */
dev->xmitwaiting = true;
/* Wait for some characters to be sent from the buffer with
* the TX interrupt enabled. When the TX interrupt is enabled,
* uart_xmitchars() should execute and remove some of the data
* from the TX buffer.
*
* NOTE that interrupts will be re-enabled while we wait for
* the semaphore.
*/
#ifdef CONFIG_SERIAL_DMA
uart_dmatxavail(dev);
#endif