Re: [PATCH v16 08/14] pvqspinlock: Implement simple paravirt support for the qspinlock

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On 05/04/2015 10:20 AM, Peter Zijlstra wrote:
I changed it to the below; I've not gotten around to compiling or even
running it yet :-(

The biggest change is the pv_hash/pv_unhash functions, which I've
rewritten to hopefully be clearer (and also hopefully not wrecked them).
I took out the cacheline sized structure which takes out that double
loop and simplifies things. I've also added some comments which
hopefully explain how/why we ended up with this exact scheme.

I've also moved the __pv_queue_spin_unlock() function to the tail, such
that we keep the 'wait'/'kick' order for both node and head.

In any case, like I just wrote on the other email, I've stuck some
things in my queue (up to and including patch 11) and if it all works
out we can continue from there.

---
Subject: pvqspinlock: Implement simple paravirt support for the qspinlock
From: Waiman Long<Waiman.Long@xxxxxx>
Date: Fri, 24 Apr 2015 14:56:37 -0400

Provide a separate (second) version of the spin_lock_slowpath for
paravirt along with a special unlock path.

The second slowpath is generated by adding a few pv hooks to the
normal slowpath, but where those will compile away for the native
case, they expand into special wait/wake code for the pv version.

The actual MCS queue can use extra storage in the mcs_nodes[] array to
keep track of state and therefore uses directed wakeups.

The head contender has no such storage directly visible to the
unlocker.  So the unlocker searches a hash table with open addressing
using a simple binary Galois linear feedback shift register.

I am fine with the change as it makes it simpler. BTW, I just saw a build error mail. That should be fixed easily with some minor edit.

Cheers,
Longman

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