Question about dec_lock_cnt method in Context class

Adam Lackorzynski adam at os.inf.tu-dresden.de
Tue Sep 16 00:13:04 CEST 2014


Hi,

On Tue Sep 09, 2014 at 17:18:16 -0400, Yuxin Ren wrote:
> There are three places where "_running_under_lock" is set to true;
> 
> First one is just in the above loop. So this code will run only after it
> goes into the loop.
> This does not help explain why the code can go into the loop.
> 
> The second is in Context::running_on_different_cpu() method in context.cpp
> file.
> if (EXPECT_FALSE(lock_cnt())
>     && EXPECT_FALSE(!mp_cas(&_running_under_lock, Mword(false),
> Mword(true))))
> return true;
> But this only happens when lock_cnt is non-zero.
> 
> The third one is in Context::need_help method in context.cpp file.
> The method is called only when a thread tries to help the lock holder.
> I think this implies that the lock holder holds a lock and as a result, its
> lock_cnt is not 0.
> 
> Just from those code, I think it is impossible for a thread that its
> "_running_under_lock" is true while its lock_cnt is 0.
> However now that the code is here, I want to know why the author add the
> "extra" check here?
> The author should have some reason to do so, right?

Yes, probably. I can just give a fuzzy answer but the new release (due
soon) will have a few changes in this area which are hopefully helping
you too.





Adam
-- 
Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/



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