Idea for Threads

[Frits van Bommel]

Martin Persenius wrote:
> 1. Before reading, check if it is locked, then wait or read.
> 2. Before writing, acquire lock, write, release.

That's not safe, there's a race condition.

Thread A: Check if locked, begin reading.
 >>> thread switch <<<
Thread B: Acquire lock, write, release.
 >>> thread switch <<<
Thread A: Continue reading.

Thread A will still have the stuff being read changing from under it...


The writing procedure needs to be modified to check if anyone's 
currently reading and, if so, wait until they're done.
This means the readers also need to mark something while they're busy. 
It'd have to be some kind of counter since multiple simultaneous readers 
are allowed.
If you want new readers to wait until a waiting writer has done it's 
thing the readers also need to actually lock something, though perhaps 
only at the beginning and end, not while they're working.

Some googling reveals that pthreads has pthread_rwlock* to implement 
this[1].


[1]: See 
http://www.die.net/doc/linux/man/man3/pthread_rwlock_init.3.html and 
related pages.