Re: wait_event_interruptible() seems non-atomic

[Jan Engelhardt]

>Hi Jan,
>
>>I would like to also lock Buffer_lock around BufRP != BufWP, but don't see a
>>way on how to accomplish this.

>This is not a problem: You compare BufRP and BufWP twice: once within
>wait_event_interruptible (without locking) and again a second test in
>your uif_read function with locking.
>You are right that the test within wait_event_interruptible is
>optimistic: a concurrent uif_read could read the new data before the
>initial uif_read has a chance to acquire the BufferLock. But it doesn't
>matter: AFAICS the test is optimistic, it can't happen that BufRP and WP
>are actually different and wait_event sleeps. And the external loop
>within uif_read() just loops if the race that you describe happened.

Yay, I see now. It takes a fair amount of looking behind the scene to get the
idea. (It's just copy-and-paste from an O'reilly book with a few
modifications.)
Yes, there is always only one way from any insn you look at, which does not
require a lock.

>Btw, could you post a link to the complete driver when asking questions?

http://ttyrpld.sf.net/ -- "kernel-2.6/rpldev.c" in the tarball.

>For example the use of down_interruptible() looks wrong to me, I'd use
>plain down().

I'd like to be able to hit Ctrl+C (in the userspace application) whenever
possible. If that's not a reason, blame the book
http://www.xml.com/ldd/chapter/book/ch03.html#t8 ("the read method" a further
down below)

BTW, the complete book is at http://www.xml.com/ldd/chapter/book/index.html


Thanks,
Jan Engelhardt
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