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Date: Tue, 17 Jan 2017 08:12:20 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Byungchul Park <byungchul.park@....com> Cc: mingo@...nel.org, tglx@...utronix.de, walken@...gle.com, boqun.feng@...il.com, kirill@...temov.name, linux-kernel@...r.kernel.org, linux-mm@...ck.org, iamjoonsoo.kim@....com, akpm@...ux-foundation.org, npiggin@...il.com Subject: Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature On Tue, Jan 17, 2017 at 11:05:42AM +0900, Byungchul Park wrote: > On Mon, Jan 16, 2017 at 04:10:01PM +0100, Peter Zijlstra wrote: > > On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote: > > > + > > > + /* > > > + * Whenever irq happens, these are updated so that we can > > > + * distinguish each irq context uniquely. > > > + */ > > > + unsigned int hardirq_id; > > > + unsigned int softirq_id; > > > > An alternative approach would be to 'unwind' or discard all historical > > events from a nested context once we exit it. > > That's one of what I considered. However, it would make code complex to > detect if pend_lock ring buffer was wrapped. I'm not sure I see the need for detecting that... > > > > After all, all we care about is the history of the release context, once > > the context is gone, we don't care. > > We must care it and decide if the next plock in the ring buffer might be > valid one or not. So I was thinking this was an overwriting ring buffer; something like so: struct pend_lock plocks[64]; unsigned int plocks_idx; static void plocks_add(..) { unsigned int idx = (plocks_idx++) % 64; plocks[idx] = ...; } static void plocks_close_context(int ctx) { for (i = 0; i < 64; i++) { int idx = (plocks_idx - 1) % 64; if (plocks[idx].ctx != ctx) break; plocks_idx--; } } Similarly for the release, it need only look at 64 entries and terminate early if the generation number is too old. static void plocks_release(unsigned int gen) { for (i = 0; i < 64; i++) { int idx = (plocks_idx - 1 - i) % 64; if ((int)(plocks[idx].gen_id - gen) < 0) break; /* do release muck */ } }
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