Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lockacquisition

[Tim Chen]

On Mon, 2013-06-24 at 16:48 -0400, Peter Hurley wrote:

> 
> Also, I haven't given a lot of thought to if preemption must be disabled
> before calling rwsem_can_spin_on_owner(). If so, wouldn't you just drop
> rwsem_can_spin_on_owner() (because the conditions tested in the loop are
> equivalent)?
> 

Not totally equivalent.  If we drop the call to rwsem_can_spin_on_owner,
we will spin when readers are holding the lock (owner is null).  
Right now we only allow writers to spin when other writers are 
holding the lock by adding the rwsem_can_spin_on_owner check.  
Letting spinning on readers held lock is tricky as
we could have a reader that sleeps and if we don't detect the case.  We
could spin for too long.

Thanks.

Tim


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