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. > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
