linux-kernel - Re: [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock

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Date:	Mon, 13 Apr 2015 17:09:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support
 for the qspinlock

On Thu, Apr 09, 2015 at 05:41:44PM -0400, Waiman Long wrote:
> >>+__visible void __pv_queue_spin_unlock(struct qspinlock *lock)
> >>+{
> >>+	struct __qspinlock *l = (void *)lock;
> >>+	struct pv_node *node;
> >>+
> >>+	if (likely(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL))
> >>+		return;
> >>+
> >>+	/*
> >>+	 * The queue head has been halted. Need to locate it and wake it up.
> >>+	 */
> >>+	node = pv_hash_find(lock);
> >>+	smp_store_release(&l->locked, 0);
> >Ah yes, clever that.
> >
> >>+	/*
> >>+	 * At this point the memory pointed at by lock can be freed/reused,
> >>+	 * however we can still use the PV node to kick the CPU.
> >>+	 */
> >>+	if (READ_ONCE(node->state) == vcpu_halted)
> >>+		pv_kick(node->cpu);
> >>+}
> >>+PV_CALLEE_SAVE_REGS_THUNK(__pv_queue_spin_unlock);
> >However I feel the PV_CALLEE_SAVE_REGS_THUNK thing belongs in the x86
> >code.
> 
> That is why I originally put my version of the qspinlock_paravirt.h header
> file under arch/x86/include/asm. Maybe we should move it back there. Putting
> the thunk in arch/x86/kernel/kvm.c didn't work when you consider that the
> Xen code also need that.

Well the function is 'generic' and belong here I think. Its just the
PV_CALLEE_SAVE_REGS_THUNK thing that arch specific. Should have live in
arch/x86/kernel/paravirt-spinlocks.c instead?
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