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SAMV7 MCAN: Lock the scheduler at one critical point to avoid a possible race condition

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This commit is contained in:
Gregory Nutt 2015-08-09 08:41:57 -06:00
parent 7a6bdf286e
commit 05aae51c89
1 changed files with 18 additions and 4 deletions
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22
arch/arm/src/samv7/sam_mcan.c
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@ -1400,8 +1400,9 @@ static void mcan_dev_lock(FAR struct sam_mcan_s *priv)
* None
*
* Assumptions:
* Called only non-interrupt logic via mcan_write. We do not have exclusive
* access to the MCAN hardware and interrupts are not disabled.
* Called only non-interrupt logic via mcan_write(). We do not have
* exclusive access to the MCAN hardware and interrupts are not disabled.
* mcan_write() does lock the scheduler for reasons noted below.
*
****************************************************************************/
@ -1477,6 +1478,12 @@ static void mcan_buffer_reserve(FAR struct sam_mcan_s *priv)
/* The FIFO is not full so the semaphore count should be greater
* than zero. If it is not, then we have missed a call to
* mcan_buffer_release(0).
*
* NOTE: Since there is no mutual exclusion, it might be possible
* that mcan_write() could be re-entered AFTER taking the semaphore
* and dropping the count to zero, but BEFORE adding the message
* to the TX FIFOQ. That corner case is handled in mcan_write() by
* locking the scheduler.
*/
else if (sval <= 0)
@ -1512,7 +1519,7 @@ static void mcan_buffer_reserve(FAR struct sam_mcan_s *priv)
}
/****************************************************************************
* Name: mcan_buffer_reserve
* Name: mcan_buffer_release
*
* Description:
* Release the semaphore, increment the semaphore count to indicate that
@ -1525,7 +1532,7 @@ static void mcan_buffer_reserve(FAR struct sam_mcan_s *priv)
* None
*
* Assumptions:
* This function is called only from the interrupt level in reponse to the
* This function is called only from the interrupt level in response to the
* complete of a transmission.
*
****************************************************************************/
@ -2499,15 +2506,22 @@ static int mcan_send(FAR struct can_dev_s *dev, FAR struct can_msg_s *msg)
* not full and cannot become full at least until we add our packet to
* the FIFO.
*
* We can't get exclusive access to MAN resource here because that
* lock the MCAN while we wait for a free buffer. Instead, the
* scheduler is locked here momentarily. See discussion in
* mcan_buffer_reserve() for an explanation.
*
* REVISIT: This needs to be extended in order to handler case where
* the MCAN device was opened O_NONBLOCK.
*/
sched_lock();
mcan_buffer_reserve(priv);
/* Get exclusive access to the MCAN peripheral */
mcan_dev_lock(priv);
sched_unlock();
/* Get our reserved Tx FIFO/queue put index */