Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

[Waiman Long]

On 02/19/2014 03:55 AM, Peter Zijlstra wrote:
> On Tue, Feb 18, 2014 at 07:58:49PM -0500, Waiman Long wrote:
> > On 02/18/2014 04:37 PM, Peter Zijlstra wrote:
> > > On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
> > > > > > +	/*
> > > > > > +	 * At the head of the wait queue now
> > > > > > +	 */
> > > > > > +	while (true) {
> > > > > > +		u32 qcode;
> > > > > > +		int retval;
> > > > > > +
> > > > > > +		retval = queue_get_lock_qcode(lock,&qcode, my_qcode);
> > > > > > +		if (retval>    0)
> > > > > > +			;	/* Lock not available yet */
> > > > > > +		else if (retval<    0)
> > > > > > +			/* Lock taken, can release the node&    return */
> > > > > > +			goto release_node;
> > > > > > +		else if (qcode != my_qcode) {
> > > > > > +			/*
> > > > > > +			 * Just get the lock with other spinners waiting
> > > > > > +			 * in the queue.
> > > > > > +			 */
> > > > > > +			if (queue_spin_trylock_unfair(lock))
> > > > > > +				goto notify_next;
> > > > > Why is this an option at all?
> > > > Are you referring to the case (qcode != my_qcode)? This condition will be
> > > > true if more than one tasks have queued up.
> > > But in no case should we revert to unfair spinning or stealing. We
> > > should always respect the queueing order.
> > >
> > > If the lock tail no longer points to us, then there's further waiters
> > > and we should wait for ->next and unlock it -- after we've taken the
> > > lock.
> > A task will be in this loop when it is already the head of a queue and is
> > entitled to take the lock. The condition (qcode != my_qcode) is to decide
> > whether it should just take the lock or take the lock&  clear the code
> > simultaneously. I am a bit cautious to use queue_spin_trylock_unfair() as
> > there is a possibility that a CPU may run out of the queue node and need to
> > do unfair busy spinning.
> No; there is no such possibility. Add BUG_ON(idx>=4) and make sure of
> it.

Yes, I could do that.

However in the generic implementation, I still need some kind of atomic 
cmpxchg to set the lock bit. I could probably just do a simple 
assignment of 1 to the lock byte in x86.

> There's simply no more than 4 contexts what can nest at any one time:
>
>    task context
>    softirq context
>    hardirq context
>    nmi context
>
> And someone contending a spinlock from NMI context should be shot
> anyway.
>
> Getting more nested spinlocks is an absolute hard fail.

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