[Bug nptl/13613] New: Cancellation is broken in single-threaded processes

[Bug nptl/13613] New: Cancellation is broken in single-threaded processes

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

             Bug #: 13613
           Summary: Cancellation is broken in single-threaded processes
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: bugdal@aerifal.cx
    Classification: Unclassified


Cancellation is not acted upon unless the process has more than one thread.
Consider the program:

#include <pthread.h>
#include <unistd.h>
int main()
{
    char c;
    pthread_cancel(pthread_self());
    read(0, &c, 1);
    return 1;
}

Being a cancellation point, read should cause the pending cancellation request
to be acted upon, and the program should terminate with status 0 as a
consequence of the last thread exiting from cancellation. However, under
glibc/NPTL, the program waits for input then terminates with status 1.

I suspect the cancellation wrapper code is incorrectly checking SINGLE_THREAD_P
and (wrongly) assuming that it implies cancellation is impossible.

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

--- Comment #1 from Rich Felker <bugdal at aerifal dot cx> 2012-03-17 20:36:37 UTC ---
Ping. Anyone looked at this?

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|drepper.fsp at gmail dot    |unassigned at sourceware
                   |com                         |dot org

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |siddhesh at redhat dot com

--- Comment #2 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-05-08 16:45:49 UTC ---
Async cancellation points get activated only when the number of threads are
greater than 1, which is why you're seeing the behaviour you see. What is your
use case anyway? I don't see why something like this should be needed at all.
Of course, I couldn't find anything in the posix definition of pthread_cancel
to not allow something like this:

http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_cancel.html

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

--- Comment #3 from Rich Felker <bugdal at aerifal dot cx> 2012-05-08 21:58:29 UTC ---
I don't have a specific usage case in mind, and I don't suspect anyone would
intentionally try to cancel the main thread in a single-threaded program
knowing it's the main thread. Where the issue is likely to matter is in code
that doesn't know it's dealing with the main thread of a single-threaded
program, e.g. in a library.

The type of application it's likely to affect is an application that does `N`
jobs in parallel by spawning `N-1` threads and using the main thread as a
worker thread. As an example, I have a parallel DNS resolver tool that does
this. If it turns out that `N` is 1, no threads get created, and the process
runs single-threaded. My DNS resolver does not have the threads using
pthread_cancel on themselves, but I could imagine moderately-contrived cases
where you might want to do this. Self-cancellation is a nice way to setup
future code to abort if it does anything that would block, while avoiding
exiting immediately. (And if you've written your program this way, you can
still call functions that are cancellation points if you temporarily block
cancellation around the call.)

In any case, glibc's behavior is non-conformant. POSIX does not make any
exceptions to the rules about how cancellation works for the special case of
single-threaded programs. (In fact the only special case I can find that POSIX
makes for single-threaded programs is that you're allowed to call
async-signal-unsafe functions after fork() in a single-threaded program.)

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[jakub at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

--- Comment #4 from Jakub Jelinek <jakub at redhat dot com> 2012-05-09 10:44:47 UTC ---
If anything, pthread_cancel should for self-cancellation (with a comment)
      THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
#ifndef TLS_MULTIPLE_THREADS_IN_TCB
      __pthread_multiple_threads = *__libc_multiple_threads_ptr = 1;
#endif
Then this will work just fine, without any overhead in the common path, except
a few insns in the already slow pthread_cancel.

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

--- Comment #5 from Rich Felker <bugdal at aerifal dot cx> 2012-05-09 14:25:09 UTC ---
I agree that's a simple and acceptable fix.

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at sourceware    |siddhesh at redhat dot com
                   |dot org                     |

--- Comment #6 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-05-09 14:56:31 UTC ---
I have submitted a patch for review:

http://sourceware.org/ml/libc-alpha/2012-05/msg00458.html

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13613

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #7 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-05-15 04:14:51 UTC ---
Fix committed to master. Thanks for reporting this.

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[Bug nptl/13613] Cancellation is broken in single-threaded processes

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=13613

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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