[Tinyos Core WG] Small gotcha in UartStream interface

[Philip Levis]

On Oct 20, 2006, at 8:23 PM, Jonathan Hui wrote:

> No race condition here. If a new byte arrives, it's handled when the
> interrupt handler executes again (of course, it does assume the
> handler is atomic). One idea is that it "suspends" the byte-by-byte
> receive, where all it's doing is putting each byte in the receive
> buffer rather than passing it up. Implicitly calling
> disableReceiveInterrupt() doesn't really do anything. The hardware
> still needs to be configured to have receive interrupts enabled,
> otherwise UartStream.receive() would not work. We could disable it
> logically, meaning that once receiveDone() returns, you have to
> explicitly enable interrupts.

What I mean is this:

-> call UartStream.enableReceiveInterrupt();
-> call UartStream.receive(.....)
<- event UartStream.receiveDone(....)
take a while to call receive again
<- event UartStream.receivedByte()

The race condition is whether you can call receive again quickly  
enough or whether receivedByte occurs. If receive() implicitly  
disables the receive interrupt, then  it's fine. Otherwise, you have  
a race, similar to the original UNIX signal() race (mmm, the mess of  
POSIX). Unless you also signal receivedByte on each byte of the  
receive()?

Phil