Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

[Ingo Molnar <mingo@elte.hu>]

* Sasha Levin <levinsasha928@gmail.com> wrote:

> I see that in liburcu there is an implementation of a rcu linked 
> list but no implementation of a rb-tree.

Another approach would be, until the RCU interactions are sorted out, 
to implement a 'big reader lock' thing that is completely lockless on 
the read side (!).

It works well if the write side is expensive, but very rare: which is 
certainly the case for these ioport registration data structures used 
in the mmio event demux fast path!

The write_lock() side signals all worker threads to finish whatever 
they are doing now and to wait for the write_unlock(). Then the 
modification can be done and the worker threads can be resumed.

This can be updated to RCU later on without much trouble.

The advantage is that this could be implemented with the existing 
thread-pool primitives straight away i think, we'd need five 
primitives:

  bread_lock();
  bread_unlock();
  bwrite_lock();
  bwrite_lock();

  brlock_init();

and a data type:

  struct brlock;

bread_lock()/bread_unlock() is basically just a compiler barrier. 
bwrite_lock() stops all (other) worker threads.
bwrite_unlock() resumes them.

That's all - should be 50 lines of code, unless i'm missing something 
:-)

Thanks,

	Ingo