Re: lock_threads

[Vladimir T. <vtz...@gm...>]

On Nov 21, 2008, at 12:14 AM, Sam Steingold wrote:
> Vladimir,
> why in lock_threads() you use begin_system_call instead of  
> begin_blocking_system_call?
> I would think that, of all system calls, xmutex_lock is the most  
> likely to block!

Because of GC and thread suspending. In order to perform GC - two  
locks should be acquired:
1. heap spinlock - so nobody can allocate new object (and thus  
probably cause again GC).
2. threads lock - while waiting for all threads to get suspended at  
safe points - we do not want new thread(s) to be spawned.

There are 3 places where lock_threads() is used without possibly  
blocking "enclosure":
1. In gc_suspend_all_threads() - after we already own the heap lock   
- in order to prevent new threads spawning.
2. suspend_thread() - in order to be sure that the thread will not  
exit while we try to suspend it (or it already has exited) - here  
more elegant solution is possible but did not want to add additional  
locks.
3. delete_thread() - it is called from a thread that terminates or  
for a thread that will never be started. The first thing that  
delete_thread() does is to ensure that anybody interested in this  
particular thread (and those are the two functions for above -  
suspending) - will consider it as already suspended - so even if we  
block here - it will not cause any harm to the GC.

All other places enclose the lock_threads() in begin_blocking_call().

Vladimir