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[[!meta copyright="Copyright © 2013 Free Software Foundation, Inc."]]

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id="license" text="Permission is granted to copy, distribute and/or modify this
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[[!meta title="cancellation points are not cancelling threads"]]

[[!tag open_issue_libpthread]]

    #include <pthread.h>
    #include <stdio.h>
    #include <sys/select.h>
    #include <unistd.h>
    
    void *f (void*foo)
    {
        char buf[128];
	//pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, NULL);
	while (1) {
	    read (0, buf, sizeof(buf));
	}
    }
    int main (void) {
        pthread_t t;
	pthread_create (&t, NULL, f, NULL);
	sleep (1);
	pthread_cancel (t);
	pthread_join (t, NULL);
	exit(0);
    }

read() is not behaving as a cancellation point, only setting the cancel
type to asynchronous permits this testcase to terminate. We do have the
pthread_setcanceltype glibc/libpthread hook in the forward structure, but we are
not using it: the LIBC_CANCEL_ASYNC macros are void, and we're not using them in
the mig msg call either.


# Provenance

## IRC, OFTC, #debian-hurd, 2013-04-15

    <paravoid> so, let me say a few things about the bug in the first place
    <paravoid> the package builds and runs a test suite
    <paravoid> the second test in the test suite blocks forever
    <paravoid> a blocked pthread_join is what I see
    <paravoid> I'm unsure why
    <paravoid> have you seen anything like it before?
    <youpi> whenever the thread doesn't actually terminate, sure
    <youpi> what is the thread usually blocked on when you cancel it?
    <paravoid> this is a hurd-specific issue
    <paravoid> works on all other arches
    <youpi> could be just that all other archs have more relaxed behavior
    <youpi> thus the question of what exactly is supposed to be happening
    <youpi> apparently it is inside a select?
    <youpi> it seems select is not cancellable here
    <pinotree> wasn't the patch you sent?
    <youpi> no, my patch was about signals
    <youpi> not cancellation
    <pinotree> k
    <youpi> (even if that could be related, of course)
    <paravoid> how did you see that?
    <paravoid> what's the equivalent of strace?
    <youpi> thread 3 is inside _hurd_select
    <paravoid> thread 1 is blocked on join
    <paravoid> but the code is
    <paravoid>     if(gdmaps->reload_thread_spawned) {
    <paravoid>         pthread_cancel(gdmaps->reload_tid);
    <paravoid>         pthread_join(gdmaps->reload_tid, NULL);
    <paravoid>     }
    <paravoid> so cancel should have killed the thread
    <youpi> cancelling a thread is a complex matter
    <youpi> there are cancellation points
    <youpi> e.g. a thread performing while(1); can't be cancelled
    <paravoid> thread 3 is just a libev event loop
    <youpi> yes, "just" calling poll, the most complex system call of unix :)
    <youpi> paravoid: anyway, don't look for a bug in your program, it's most
      likely a bug in glibc, thanks for the report
    <paravoid> I think it all boils down to a problem cancelling a thread in
      poll()
    <youpi> yes
    <youpi> paravoid: ok, actually with the latest libc it does work
    <paravoid> oh?
    <youpi> where latest = not uploaded yet :/
    <paravoid> did you test this on exodar?
    <youpi> pinotree: that's the libpthread_cancellation.diff I guess
    <paravoid> because I commented out the join :)
    <youpi> paravoid:  in the root, yes
    <youpi> well, I tried my own program
    <paravoid> oh, okay
    <youpi> which is indeed hanging inside select (or just read) in the chroot
    <youpi> but not in the root
    <pinotree> ah, richard's patch
    <paravoid> url?
    <youpi> I've installed the build-dep in the root, if you want to try
    <paravoid> strange that root is newer than the chroot :)
    <youpi> paravoid: it's the usual eglibc debian source
    <paravoid> tried in root, still fails
    <youpi> could you keep the process running?
    <paravoid> done
    <youpi> Mmm, but the thread running gdmaps_reload_thread never set the
      cancel type to async?
    <youpi> that said I guess read and select are supposed to be cancellation
      points
    <youpi> thus cancel_deferred should be working, but they are not
    <youpi> it seems it's cancellation points which have just not been
      implemented
    <youpi> (they happen to be one of the most obscure things in posix)


## IRC, freenode, #hurd, 2013-04-15

    <youpi> but yes, there is still an issue, with PTHREAD_CANCEL_DEFERRED
    <youpi> how calls like read() or select() are supposed to test
      cancellation?
    <pinotree> iirc there are the LIBC_CANCEL_* macros in glibc
    <pinotree> eg sysdeps/unix/sysv/linux/pread.c
    <youpi> yes
    <youpi> but in our libpthredaD?
    <pinotree> could it be we lack the libpthread → glibc bridge of
      cancellation stuff?
    <youpi> we do have pthread_setcancelstate/type forwards
    <youpi> but it seems the default LIBC_CANCEL_ASYNC is void
    <pinotree> i mean, so when you cancel a thread, you can get that cancel
      status in libc proper, just like it seems done with LIBC_CANCEL_* macros
      and nptl
    <youpi> as I said, the bridge is there
    <youpi> we're just not using it in glibc
    <youpi> I'm writing an open_issues page


### IRC, freenode, #hurd, 2013-04-16

    <braunr> youpi: yes, we said some time ago that it was lacking