MTres19 commented on a change in pull request #2641:
URL: https://github.com/apache/incubator-nuttx/pull/2641#discussion_r774280528
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File path: include/nuttx/can/can.h
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@@ -491,7 +491,13 @@ begin_packed_struct struct can_msg_s
struct can_rxfifo_s
{
- sem_t rx_sem; /* Counting semaphore */
+ /* Binary semaphore. Indicates whether FIFO is available for reading
+ * AND not empty. Only take this sem inside a critical section to guarantee
+ * exclusive access to both the semaphore and the head/tail FIFO indices.
+ */
+
+ sem_t rx_sem;
Review comment:
No; `rx_sem` is not used as a lock because the ISR (which calls
`can_receive()`) can edit the FIFO whenever it wants as long as interrupts are
enabled. `rx_sem` exists to allow `can_receive()` to wake up a reader thread,
but the reader thread then has exclusive access only because it uses a critical
section, not because it took the semaphore.
See also my reply above.
Actually, `rx_sem` was a binary semaphore before too, but "0" and "1" didn't
have any defined meaning because it was initialized to 1 and then immediately
decremented in a loop in `can_read()` and `can_poll()`
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