Re: [Xen-devel] [PATCH 01/11] qspinlock: A simple generic 4-byte queue spinlock

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Jun 23, 2014 at 06:26:22PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 17, 2014 at 04:05:31PM -0400, Konrad Rzeszutek Wilk wrote:
> > > + * The basic principle of a queue-based spinlock can best be understood
> > > + * by studying a classic queue-based spinlock implementation called the
> > > + * MCS lock. The paper below provides a good description for this kind
> > > + * of lock.
> > > + *
> > > + * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
> > > + *
> > > + * This queue spinlock implementation is based on the MCS lock, however 
> > > to make
> > > + * it fit the 4 bytes we assume spinlock_t to be, and preserve its 
> > > existing
> > > + * API, we must modify it some.
> > > + *
> > > + * In particular; where the traditional MCS lock consists of a tail 
> > > pointer
> > > + * (8 bytes) and needs the next pointer (another 8 bytes) of its own 
> > > node to
> > > + * unlock the next pending (next->locked), we compress both these: {tail,
> > > + * next->locked} into a single u32 value.
> > > + *
> > > + * Since a spinlock disables recursion of its own context and there is a 
> > > limit
> > > + * to the contexts that can nest; namely: task, softirq, hardirq, nmi, 
> > > we can
> > > + * encode the tail as and index indicating this context and a cpu number.
> > > + *
> > > + * We can further change the first spinner to spin on a bit in the lock 
> > > word
> > > + * instead of its node; whereby avoiding the need to carry a node from 
> > > lock to
> > > + * unlock, and preserving API.
> > 
> > You also made changes (compared to the MCS) in that the unlock path is not
> > spinning waiting for the successor and that the job of passing the lock
> > is not done in the unlock path either.
> > 
> > Instead all of that is now done in the path of the lock acquirer logic. 
> > 
> > Could you update the comment to say that please?
> 
> I _think_ I know what you mean.. So that is actually implied by the last

You do :-)

> paragraph, but I suppose I can make it explicit; something like:
> 
>   *
>   * Another way to look at it is:
>   *
>   *  lock(tail,locked)
>   *    struct mcs_spinlock node;
>   *    mcs_spin_lock(tail, &node);
>   *    test-and-set locked;
>   *    mcs_spin_unlock(tail, &node);
>   *
>   *  unlock(tail,locked)
>   *    clear locked
>   *
>   * Where we have compressed (tail,locked) into a single u32 word.
> 
> 

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