sem_wait() is a cancellation point in libkse?

[Daniel Eischen]

On Tue, 20 Jan 2004, Craig Rodrigues wrote:

> Hi,
> 
> I am trying to learn about pthread cancellation points,
> and am reading "Multithreaded Programming with PThreads" by
> Bil Lewis.  In chapter 10 of this book, it states
> that sem_wait() should be implemented as a pthread 
> cancellation point.
> 
> I am not sure that sem_wait() is implemented as
> a cancellation point in libkse.  
> 
> I looked again at some of the Linux NPTL tests
> which I ported to FreeBSD:
> 
> http://crodrigues.org/freebsd/nptl-tests.tar.gz
> 
> 
> The tst-cancel12 test seems to be failing.
> In this test, a thread is spawned with
> pthread_create(), and then soon after
> pthread_create(), pthread_cancel() is called
> on this thread.
> 
> Inside the thread, sem_wait() is called.
> The test is expecting that the thread should
> immediately exit at the sem_wait() call because
> sem_wait() should be a cancellation point.
> However, the thread does not exist after calling
> sem_wait(), so the test fails.
> 
> I see that in libkse's thr_sem.c, that there
> is a _sem_wait() which looks like it is doing
> cancellation stuff, but it doesn't
> seem to be working.
> 
> Am I misunderstanding how this should work?

libc uses strong symbols for sem_wait which override
the weak symbols that libpthread is using.  The sem_wait
in libc isn't a cancellation point yet.  I think the
sem_foo stuff in libc should be refactored to just make
the system calls (like semop/semctl) and not know about
waiting threads.  The threads libraries can wrap them
if they want to provide cancellation points and faster
userland thread switches for (process scope) thread
waits/wakeups.

In short, we know about this.

-- 
Dan Eischen