c - pthread_mutex_lock locks, but no owner is set

Question

I've been working on this one for a few days -

As a background, I'm working on taking a single-threaded C program and making it multi-threaded. I have recently discovered a new deadlock case, but when I look at the mutex in gdb I see that

__lock=2 yet __owner=0

This is not a recursive mutex. Has anyone seen this? The program I'm working on is a daemon and this case only happens after executing at a high-throughput rate for over 20 minutes (approximately) and then relaxing the load. If you have any ideas I'd be grateful.

Edit - I neglected to mention that all of my other threads are idle at this time.

Cheers

Answer 1

This is to be expected. A normal (non-recursive, non-errorchecking) mutex has no need to store its owner, and some time can be saved skipping the step of looking up the caller's thread id. (This makes little difference on x86 but can be a huge difference on platforms like MIPS with broken ABIs, where there is no thread register and getting the thread id incurs a fault into kernelspace.)

The deadlock you're seeing it almost certainly due either to the thread trying to lock a mutex it already holds, or an actual logic error where two or more threads are each waiting for mutexes the other holds.