Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers

[Szabolcs Nagy]

* Rich Felker <dalias@xxxxxxxx> [2016-03-10 19:48:59 -0500]:
> On Fri, Mar 11, 2016 at 01:18:54AM +0100, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@xxxxxxxx> [2016-03-10 18:28:20 -0500]:
> > > On Thu, Mar 10, 2016 at 07:03:31PM +0100, Ingo Molnar wrote:
> > > > 
> > > > The sticky signal is only ever sent when the thread is in cancellable state - and 
> > > > if the target thread notices the cancellation request before the signal arrives, 
        ^^^^^^...
> > > > it first waits for its arrival before executing any new system calls (as part of 
        ^^^^^^...
> > > > the teardown, etc.).
> > > > 
> > > > So the C library never has to do complex work with a sticky signal pending.
> > > > 
> > > > Does that make more sense to you?
> > > 
> > > No, it doesn't work. Cancellability of the target thread at the time
> > > of the cancellation request (when you would decide whether or not to
> > > send the signal) has no relation to cancellability at the time of
> > > calling the cancellation point. Consider 2 threads A and B and the
> > > following sequence of events:
> > > 
> > > 1. A has cancellation enabled
> > > 2. B calls pthread_cancel(A) and sets sticky pending signal
> > > 3. A disables cancellation
> > > 4. A calls cancellation point and syscall wrongly gets interrupted
> > > 
> > > This can be solved with more synchronization in pthread_cancel and
> > > pthread_setcancelstate, but it seems costly. pthread_setcancelstate
> > > would have to clear pending sticky cancellation signals, and any
> > > internal non-cancellable syscalls would have to be made using the same
> > > mechanism (effectively calling pthread_setcancelstate). A naive
> > > implementation of such clearing would involve a syscall itself,
> > 
> > i think a syscall in setcancelstate in case of pending sticky signal
> > is not that bad given that cancellation is very rarely used.
> 
> I agree, but it's not clear to me whether you could eliminate syscalls
> in the case where it's not pending, since AS-safe lock machinery is
> hard to get right. I don't see a way it can be done with just atomics
> because the syscall that sends the signal cannot be atomic with the
> memory operating setting a flag, which suggests a lock is needed, and
> then there are all sorts of issues to deal with.
> 

i think this is not a problem and the above marked text hints for
a solution: just call pause() to wait for the sticky signal if
self->cancelstate indicates that there is one comming or pending.

t->cancelstate always have to be atomically modified but sending
the sticky signal can be delayed (does not have to be atomic with
the memory op).

(of course there migth be other caveats and it certainly needs
more atomic ops and more state than the current design)