Proposal: a revoke() system call

[Robert Watson]

On Mon, 7 Jul 2008, Sergey Babkin wrote:

>> On Sun, 6 Jul 2008, Sergey Babkin wrote:
>>
>>>  int revoke(int fd, int flags)
>>
>> Seems like that conflicts with our existing revoke(2) system call.  You 
>> could
>
> Aha, I guess when I've checked, I've looked at a real old version of 
> FreeBSD. Sure, the name can be changed.

I won't point you at the HISTORY section of the revoke(2) system call then 
:-).

>> achieve something of the same end by opening /dev/null and then dup2()'ing 
>> to the file descriptor you want to revoke, perhaps?  Right now there's a 
>> known
>
> That's a great idea. I haven't thought about it. It should do everything.

Right, and possibly this means that no additional kernel support is required 
-- we just make it a libc or libutil interface.

>> issue that calling close(2) on a socket from one thread doesn't interrupt a 
>> socket in a blocking I/O call from another thread -- you first have to call 
>> shutdown(2), and then close(2).  This has caused problems for Java in the 
>> past, but I'm not sure that it's really a bug given that it's not 
>> unreasonable behavior not rejected by the spec :-).
>
> Maybe I'll see if I can fix that.

Well, fixing this is easy -- instead of holding a reference to the file 
descriptor over the system call, hold a reference to the socket.  The problem 
with that is that it creates a lot more contention on the socket locks when 
the reference count is dropped, not to mention more locking operations.  This 
can be fixed but requires quite a lot of work, whereas this rather minor 
semantic issue is a non-problem in practice.  I do have dealing with this 
reference issue on my todo list, but it's very low on the list because there 
are lots of other areas where we can significantly improve performance or 
semantics more easily and more quickly :-).

Robert N M Watson
Computer Laboratory
University of Cambridge