with a new implementation that has a mostly reentrant "addchar"
routine, supports multiple message buffers in the kernel, and hides
the implementation details from callers.
The new code uses a kind of sequence number to represend the current
read and write positions in the buffer. This approach (suggested
mainly by bde) permits the read and write pointers to be maintained
separately, which reduces the number of atomic operations that are
required. The "mostly reentrant" above refers to the way that while
it is now always safe to have any number of concurrent writers,
readers could see the message buffer after a writer has advanced
the pointers but before it has witten the new character.
Discussed on: freebsd-arch
This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
Fixed the following bugs:
- the buffer was reprinted endlessly when msg.bufx == 0 and (for a
different reason) when msg.bufx == 1.
- the last byte of the buffer wasn't printed except in the the infinite
loop cases.
- the comment about walking the buffer didn't match the (correct) code.
- minor -Wall and style bugs.
Not fixed:
- excessive newline processing which hid the non-printing of the last
byte of the buffer.
