linux-kernel - Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150713123136.GX18673@twins.programming.kicks-ass.net>
Date:	Mon, 13 Jul 2015 14:31:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

On Mon, Jul 13, 2015 at 02:02:02PM +0200, Peter Zijlstra wrote:
> On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote:
> > @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node)
> >  			pv_wait(&pn->state, vcpu_halted);
> >  
> >  		/*
> > -		 * Reset the vCPU state to avoid unncessary CPU kicking
> > +		 * Reset the state except when vcpu_hashed is set.
> >  		 */
> > -		WRITE_ONCE(pn->state, vcpu_running);
> > +		cmpxchg(&pn->state, vcpu_halted, vcpu_running);
> 
> Why? Suppose we did get advanced into the hashed state, and then get a
> (spurious) wakeup, this means we'll observe our ->locked == 1 condition
> and fall out of pv_wait_node().
> 
> We'll then enter pv_wait_head(), which with your modification:
> 
> > @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
> >  {
> >  	struct pv_node *pn = (struct pv_node *)node;
> >  	struct __qspinlock *l = (void *)lock;
> > -	struct qspinlock **lp = NULL;
> > +	struct qspinlock **lp;
> >  	int loop;
> >  
> > +	/*
> > +	 * Initialize lp to a non-NULL value if it has already been in the
> > +	 * pv_hashed state so that pv_hash() won't be called again.
> > +	 */
> > +	lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1
> > +						   : NULL;

Because that ^

> >  	for (;;) {
> > +		WRITE_ONCE(pn->state, vcpu_running);
> 
> Will instantly and unconditionally write vcpu_running.
> 
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/