Re: [PATCH] compatibility syscall layer (lets try again)

From: george anzinger (
Date: Tue Dec 10 2002 - 18:07:37 EST

Daniel Jacobowitz wrote:
> 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.

I seem to recall working on a debugger in the distant past
and put a lock in it that did not allow it to run a debuggee
function while the debugee was in a system call. It seems
to me that is is begging for problems and is not that hard
for gdb/etc to prevent.

Daniel, what to you think?

> 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

George Anzinger
Preemption patch:
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