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

[Peter Zijlstra <peterz@infradead.org>]

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
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.