Clark Williams wrote:
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Darren Hart wrote:
On Thu, 2008-05-29 at 16:46 -0700, Stephen Hemminger wrote:
On Thu, 29 May 2008 23:41:55 +0000
Darren Hart <dvhltc@xxxxxxxxxx> wrote:
I find I need to be able to break out of the blocked state while waiting
to acquire a pthread_mutex. I'm using PI mutexes and want to continue
to do so. As I understand it, I can't use a signal to break out of the
lock as the man pages states:
"If a signal is delivered to a thread waiting for a mutex, upon return
from the signal handler the thread shall resume waiting for the mutex
as if it was not interrupted."
and that pthread_mutex_lock will not return EINTR. I had considered
using cond variables, but I don't think they will provide the same PI
behavior (since the threads are not blocked on the mutex while awaiting
the pthread_cond_signal - right?).
I'm sure I'm not the first to want to do this, does anyone know of a
common best practice for accomplishing such a thing?
setjmp/longjmp?
Stephen,
Hrmm... I confess to not having made use of them before. I read up a
bit, and don't see right off how they would help in this situation. I
assume you're suggesting the longjmp be used in a signal handler while
the thread is blocked on the mutex - where would it jump to, and how
would I ensure the integrity of the pthread_mutex structure (which
thinks my thread is waiting for it) ?
I don't see any way to recover from longjmp'ing out of a pthread_mutex_lock(). Maybe
using robust futexes and killing the thread that was blocked?
I think it would be better to re-designed the code to use pthread_mutex_trylock()
instead, so it doesn't block. That way you don't have a mutex in some wacky state.
The easiest thing to do (if you want to modify the syscall code) is to
change the return(ERESTARTNOINTR) by return(ERESTARTSYS) in
futex_lock_pi (futex.c) and in your application, play with sigaction's
SA_RESTART flag.
Beware that pthread_mutex_trylock (glibc) might still return 0.
Gilles.
Clark
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