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SubjectRe: [PATCH] compatibility syscall layer (lets try again)
Linus Torvalds wrote:
>
> On Wed, 4 Dec 2002, Jim Houston wrote:
> >
> > Agreed! In my alternative version of the Posix timers patch, I avoid
> > calling do_signal() from clock_nanosleep by using a variant of the
> > existing ERESTARTNOHAND mechanism. The problem I ran into was that I
> > could not tell on entry to clock_nanosleep if it was a new call or
> > an old one being restarted.
>
> Restarting has other problems too, namely how to save off the partial
> results.
>
> > I solved this by adding a new
> > ERESTARTNANOSLP error code and making a small change in do_signal().
> > The handling of ERESTARTNANOSLP is the same as ERESTARTNOHAND but also
> > sets a new flag in the task_struct before restarting the system call.
>
> The problem I see with this is that the signal handler can do a
> "siglongjump()" out of the regular path, and the next system call may well
> be a _new_ nanosleep() that has nothing to do with the old one. And
> realizing that it's _not_ a restarted one is interesting.
>
> A better and more flexible approach would be to not restart the same
> system call with the same parameters, but having some way of telling
> do_signal to restart with new parameters and a new system call number.
>
> For example, it shouldn't be impossible to have an interface more akin to
>
> ...
> thread_info->restart_block.syscall = __NR_nanosleep_restart;
> thread_info->restart_block.arg0 = timeout + jiffies; /* absolute time */
> return -ERESTARTSYS_RESTARTBLOCK;
>
> where the signal stack stuff re-writes not just eip (like the current
> restart logic does), but also rewrites the system call number and the
> argument registers.
>
> This way you can get a truly restartable system call, because the
> arguments really need to be fundamentally changed (the restarted system
> call had better have _absolute_ time, not relative time, since we don't
> know how much time passed before it got restarted).
>
> Linus

Hi Linus,

The general solution you propose sounds nice but I have a feeling
the implementation would get ugly. It is hard enough to back up the
pc. I hate to think where the arguments are on some machines.

I think that "siglongjump()" is not a problem. My change to
do_signal() only sets the flag indicating a restart at the same time
it backs up the pc to restart the system call. I don't see a path
where the user code gets control before we're back at clock_nanosleep.

I'm saving the information to restart the nanosleep in the task_struct.
I have a pre-allocated timer which I leave running. When I get
into nanosleep for the restart, I just have to check if the timer has
already expired and, if not, go back to sleep. To calculate the
remaining time I also save an un-rounded copy of the absolute expiry
time (also in the task_struct).

Jim Houston - Concurrent Computer Corp.
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