cvs commit: src/sys/kern kern_thr.c syscalls.master src/sys/sys thr.h

[Robert Watson]

On Sat, 18 Aug 2007, Tijl Coosemans wrote:

> To be honest, I'm not entirely fond of adding this syscall either, but from 
> an engineering point of view, it's really the easiest and cheapest solution 
> right now.
>
> POSIX message queues aren't needed. Sigqueue() with sigval set to the thread 
> id would be enough. Then whichever thread receives the signal can either 
> handle it or dispatch it using thr_kill().
>
> Sigqueue() doesn't exist in 6-STABLE though and as I understood will never 
> be MFC'd, because it involves too many other things (signal queueing, 
> ksiginfo,...), whereas thr_kill2() is entirely isolated and trivial to 
> merge.
>
> Perhaps it should be stressed in documentation that this syscall is only 
> meant for Wine and to be removed at some point when better solutions become 
> available. I want to test this sigqueue solution some more, because the 
> indirection in signal delivery causes trouble when a thread signals itself. 
> I guess this would have to be special-cased somehow then.

As a general rule, removing system calls is discouraged as it breaks backward 
compatibility allowing newer kernels to run older applications, so I think we 
should avoid adding a system call with the intent to remove it in mind at the 
time it's added :-).  While thr_kill2() is imperfect from several 
perspectives, it is certainly a minimalist and logical construction that can 
be easily understood, and that's pretty valuable.

Robert N M Watson
Computer Laboratory
University of Cambridge