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Peter Anvin" , LKML , Ingo Molnar , nstange@suse.de, syzkaller-bugs@googlegroups.com, henrik@austad.us, Tommaso Cucinotta , Claudio Scordino , Daniel Bristot de Oliveira Subject: Re: INFO: rcu detected stall in do_idle Message-ID: <20181028083355.GA30631@localhost.localdomain> References: <20181016153608.GH9130@localhost.localdomain> <20181018082838.GA21611@localhost.localdomain> <20181018122331.50ed3212@luca64> <20181018104713.GC21611@localhost.localdomain> <20181018130811.61337932@luca64> <20181019113942.GH3121@hirez.programming.kicks-ass.net> <20181019225005.61707c64@nowhere> <20181024120335.GE29272@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/10/18 12:16, Dmitry Vyukov wrote: > On Wed, Oct 24, 2018 at 1:03 PM, Juri Lelli wrote: > > > > On 19/10/18 22:50, luca abeni wrote: > > > On Fri, 19 Oct 2018 13:39:42 +0200 > > > Peter Zijlstra wrote: > > > > > > > On Thu, Oct 18, 2018 at 01:08:11PM +0200, luca abeni wrote: > > > > > Ok, I see the issue now: the problem is that the "while > > > > > (dl_se->runtime <= 0)" loop is executed at replenishment time, but > > > > > the deadline should be postponed at enforcement time. > > > > > > > > > > I mean: in update_curr_dl() we do: > > > > > dl_se->runtime -= scaled_delta_exec; > > > > > if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) { > > > > > ... > > > > > enqueue replenishment timer at dl_next_period(dl_se) > > > > > But dl_next_period() is based on a "wrong" deadline! > > > > > > > > > > > > > > > I think that inserting a > > > > > while (dl_se->runtime <= -pi_se->dl_runtime) { > > > > > dl_se->deadline += pi_se->dl_period; > > > > > dl_se->runtime += pi_se->dl_runtime; > > > > > } > > > > > immediately after "dl_se->runtime -= scaled_delta_exec;" would fix > > > > > the problem, no? > > > > > > > > That certainly makes sense to me. > > > > > > Good; I'll try to work on this idea in the weekend. > > > > So, we (me and Luca) managed to spend some more time on this and found a > > few more things worth sharing. I'll try to summarize what we have got so > > far (including what already discussed) because impression is that each > > point might deserve a fix or at least consideration (just amazing how a > > simple random fuzzer thing can highlight all that :). > > 1. Fuzzing finds bugs in any code. Always. > If a code wasn't fuzzed, there are bugs. > > 2. This fuzzer is not so simple ;) Indeed! I meant that it's amazing how the fuzzer was able to forge a relatively simple reproducer that highlighted the problem.