[PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks
tj at kernel.org
Thu Jan 24 06:57:40 EST 2013
On Thu, Jan 24, 2013 at 01:03:52AM +0530, Srivatsa S. Bhat wrote:
> Hmm.. I split it up into steps to help explain the reasoning behind
> the code sufficiently, rather than spring all of the intricacies at
> one go (which would make it very hard to write the changelog/comments
> also). The split made it easier for me to document it well in the
> changelog, because I could deal with reasonable chunks of code/complexity
> at a time. IMHO that helps people reading it for the first time to
> understand the logic easily.
I don't know. It's a judgement call I guess. I personally would much
prefer having ample documentation as comments in the source itself or
as a separate Documentation/ file as that's what most people are gonna
be looking at to figure out what's going on. Maybe just compact it a
bit and add more in-line documentation instead?
> > The only two options are either punishing writers or identifying and
> > updating all such possible deadlocks. percpu_rwsem does the former,
> > right? I don't know how feasible the latter would be.
> I don't think we can avoid looking into all the possible deadlocks,
> as long as we use rwlocks inside get/put_online_cpus_atomic() (assuming
> rwlocks are fair). Even with Oleg's idea of using synchronize_sched()
> at the writer, we still need to take care of locking rules, because the
> synchronize_sched() only helps avoid the memory barriers at the reader,
> and doesn't help get rid of the rwlocks themselves.
Well, percpu_rwlock don't have to use rwlock for the slow path. It
can implement its own writer starving locking scheme. It's not like
implementing slow path global rwlock logic is difficult.
> CPU 0 CPU 1
> write_lock(&rwlock) //spins, because CPU 0
> //has acquired the lock for read
> What happens here? Does CPU 0 start spinning (and hence deadlock) or will
> it continue realizing that it already holds the rwlock for read?
I don't think rwlock allows nesting write lock inside read lock.
read_lock(); write_lock() will always deadlock.
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