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[209.85.220.65]) by mx.google.com with SMTPS id m200sor4853185itm.33.2019.03.12.10.10.49 for (Google Transport Security); Tue, 12 Mar 2019 10:10:49 -0700 (PDT) Received-SPF: pass (google.com: domain of dvyukov@google.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b="hSBUd/eG"; spf=pass (google.com: domain of dvyukov@google.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dvyukov@google.com; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=q63ookcgnvhAqs3Sfl5pVKDmKu933OB0+OqZ26Ou1RY=; b=hSBUd/eG9bU9hkcXf07Vnzfvi70ZPJSBcBfruaCjFYM14tdDKN1568MVLHVkp1/rzg +6Kvr4CytIiREYS1zYlJHoH0flAM+XoTrGW0QCnwrTFi8ah/X4azfw5jprA37easCbb6 WWKnUm+2oRUpKiOdwawihqIoCdbHuur985R5Kkka3cKAsMMJZbuwhwEvkcBX7iQcpmff Ltj6D7ENtQA15Jtc0daYUa/UPPmEAq+7Q7HYNkrp8zPyXJl1seIegJbM0EEuiUPKUWsi h6I5mMiXJmk0x9sKsteaUVj7+VdBvxAvjIcz+jlPIwTaTcR9NVX/luULEMgqgAncsgAa nynw== X-Google-Smtp-Source: APXvYqwqCfJYHXCtfsLZSl2M8UsGhzjYvPKuIRwXqEZvL3LVq/MLO/l88WdygX5M9kEtHiE4/PMhCh92IpnBJ8mAObU= X-Received: by 2002:a24:3b01:: with SMTP id c1mr2440750ita.144.1552410649381; Tue, 12 Mar 2019 10:10:49 -0700 (PDT) MIME-Version: 1.0 References: <00000000000010b2fc057fcdfaba@google.com> <0000000000008c75b50583ddb5f8@google.com> <20190312040829.GQ2217@ZenIV.linux.org.uk> In-Reply-To: <20190312040829.GQ2217@ZenIV.linux.org.uk> From: Dmitry Vyukov Date: Tue, 12 Mar 2019 18:10:37 +0100 Message-ID: Subject: Re: INFO: rcu detected stall in sys_sendfile64 (2) To: Al Viro Cc: syzbot , David Airlie , Andrew Morton , Amir Goldstein , Chris Wilson , "Darrick J. Wong" , Dave Chinner , DRI , eparis@redhat.com, Johannes Weiner , Hugh Dickins , intel-gfx , Jan Kara , Jani Nikula , Joonas Lahtinen , Souptick Joarder , LKML , Linux-MM , Ingo Molnar , mszeredi@redhat.com, Tetsuo Handa , Peter Zijlstra , Rodrigo Vivi , syzkaller-bugs , Matthew Wilcox Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 12, 2019 at 5:08 AM Al Viro wrote: > > On Mon, Mar 11, 2019 at 08:59:00PM -0700, syzbot wrote: > > syzbot has bisected this bug to: > > > > commit 34e07e42c55aeaa78e93b057a6664e2ecde3fadb > > Author: Chris Wilson > > Date: Thu Feb 8 10:54:48 2018 +0000 > > > > drm/i915: Add missing kerneldoc for 'ent' in i915_driver_init_early > > > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=13220283200000 > > start commit: 34e07e42 drm/i915: Add missing kerneldoc for 'ent' in i915.. > > git tree: upstream > > final crash: https://syzkaller.appspot.com/x/report.txt?x=10a20283200000 > > console output: https://syzkaller.appspot.com/x/log.txt?x=17220283200000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=abc3dc9b7a900258 > > dashboard link: https://syzkaller.appspot.com/bug?extid=1505c80c74256c6118a5 > > userspace arch: amd64 > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12c4dc28c00000 > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15df4108c00000 > > > > Reported-by: syzbot+1505c80c74256c6118a5@syzkaller.appspotmail.com > > Fixes: 34e07e42 ("drm/i915: Add missing kerneldoc for 'ent' in > > i915_driver_init_early") > > Umm... Might be a good idea to add some plausibility filters - it is, > in theory, possible that adding a line in a comment changes behaviour > (without compiler bugs, even - playing with __LINE__ is all it would > take), but the odds that it's _not_ a false positive are very low. Thanks for pointing this out. I've started collecting all such cases, so that we are able to draw broader conclusions later: https://github.com/google/syzkaller/issues/1051 added for this one: ========= A mix of problems: unrelated bug triggered by the same repro ("WARNING: ODEBUG bug in netdev_freemem"); lots of infrastructure failures ("failed to copy test binary to VM"); also the original failure seems to be flaky. All this contributed to pointing to a random commit. Al Viro points out that the commit only touches comments, so we could mark the end result as suspicious. ========= The infrastructure problems is definitely something we need to fix ("failed to copy test binary to VM") (currently the machine hangs periodically with lots of time consumed by dmcrypt, but I don't know if it's related or not yet). Re the comment-only changes, I would like to see more cases where it would help before we start creating new universes for this. We could parse sources with clang to understand that a change was comment-only, but I guess kernel is mostly broken with clang throughout history....