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SubjectRe: [PATCH] compatibility syscall layer (lets try again)
On Fri, Dec 06, 2002 at 09:57:08AM -0800, Linus Torvalds wrote:
> I just pushed my version of the system call restart code to the BK trees.
> It's losely based on Georges code, but subtly different. Also, I didn't
> actually update any actual system calls to use it, I just did the
> infrastructure.
> Non-x86 architectures need to be updated to work with this: they need to
> update their thread structures, the additional do_signal() support in
> their signal.c, and add the actual system call itself somewhere. For x86,
> this was about 15 lines of changes.
> The basic premise is very simple: if you want to restart a system call,
> you can do
> restart = &current_thread()->restart_block;
> restart->fn = my_continuation_function;
> restart->arg0 = my_arg0_for_continuation;
> restart->arg1 = my_arg1_for_continuation;
> ..
> which will cause the system call to either return -EINTR (if a signal
> handler was actually invoced) or for "benign" signals (SIGSTOP etc) it
> will end up restarting the system call at the continuation function (with
> the "restart" block as the argument).
> We could extend this to allow restarting even over signal handlers, but
> that would have some re-entrancy issues (ie what happens if a signal
> handler itself wants to use a system call that may want restarting), so at
> least for now restarting is only done when no handler is invoced (*).
> Linus
> (*) The nesting case is by no means impossible to handle gracefully
> (adding a "restart even if handler is called" error number and returning
> -EINTR if nesting, for example), but I don't know of any system calls that
> would really want to try to restart anyway, so..

Well, here's something to consider. This isn't entirely hypothetical;
there are test cases in GDB's regression suite that cover nearly this.

Suppose a process is sleeping for an hour. The user wants to see what
another thread is doing, so he hits Control-C; the thread which happens
to be reported as 'current' is the one that was in nanosleep(). It
used to be that when he said continue, the nanosleep would return; now
hopefully it'll continue. Great! But this damnable user isn't done
yet. He wants to look at one of his data structures. He calls a
debugging print_foo() function from GDB. He realizes he left a
sleep-for-a-minute nanosleep call in it and C-c's again. Now we have
two interrupted nanosleep calls and the application will never see a
signal to interrupt either of them; he says "continue" twice and
expects to get back to his hour-long sleep.

Note that I'm not saying we _need_ to support this, mind :) It's a
little pathological.

Another thing that annoys me slightly about this is that we mess with
the value in orig_eax etc. Now a debugger would have to look at the
instruction stream to figure out what the syscall was that we're
stopped in, reliably. Not a big deal.

Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer
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