Messages in this thread | | | Date | Wed, 1 Mar 2017 14:43:23 +0900 | From | Byungchul Park <> | Subject | Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature |
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On Tue, Feb 28, 2017 at 02:40:18PM +0100, Peter Zijlstra wrote: > > +static int commit_xhlocks(struct cross_lock *xlock) > > +{ > > + struct task_struct *curr = current; > > + struct hist_lock *xhlock_c = xhlock_curr(curr); > > + struct hist_lock *xhlock = xhlock_c; > > + > > + do { > > + xhlock = xhlock_prev(curr, xhlock); > > + > > + if (!xhlock_used(xhlock)) > > + break; > > + > > + if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id)) > > + break; > > + > > + if (same_context_xhlock(xhlock) && > > + before(xhlock->prev_gen_id, xlock->hlock.gen_id) && > > + !commit_xhlock(xlock, xhlock)) > > + return 0; > > + } while (xhlock_c != xhlock); > > + > > + return 1; > > +} > > So I'm still struggling with prev_gen_id; is it an optimization or is it > required for correctness?
It's an optimization, but very essential and important optimization.
in hlocks[] ------------ A gen_id (4) --+ | previous gen_id B gen_id (3) <-+ C gen_id (3) D gen_id (2) oldest -> E gen_id (1)
in xhlocks[] ------------ ^ A gen_id (4) prev_gen_id (3: B's gen id) | B gen_id (3) prev_gen_id (3: C's gen id) | C gen_id (3) prev_gen_id (2: D's gen id) | D gen_id (2) prev_gen_id (1: E's gen id) | E gen_id (1) prev_gen_id (NA)
Let's consider the case that the gen id of xlock to commit is 3.
In this case, it's engough to generate 'the xlock -> C'. 'the xlock -> B' and 'the xlock -> A' are unnecessary since it's covered by 'C -> B' and 'B -> A' which are already generated by original lockdep.
I use the prev_gen_id to avoid adding this kind of redundant dependencies. In other words, xhlock->prev_gen_id >= xlock->hlock.gen_id means that the previous lock in hlocks[] is able to handle the dependency on its commit stage.
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