From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/5839 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: My current understanding of cond var access restrictions Date: Thu, 14 Aug 2014 10:00:04 +0200 Message-ID: <1408003204.4951.92.camel@eris.loria.fr> References: <20140813212358.GA25429@brightrain.aerifal.cx> <1407972025.4951.73.camel@eris.loria.fr> <20140814061009.GA6599@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-wfvJQUxkvr7RRJBIeucI" X-Trace: ger.gmane.org 1408003243 14965 80.91.229.3 (14 Aug 2014 08:00:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 14 Aug 2014 08:00:43 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-5845-gllmg-musl=m.gmane.org@lists.openwall.com Thu Aug 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.4.4-3 Xref: news.gmane.org gmane.linux.lib.musl.general:5839 Archived-At: --=-wfvJQUxkvr7RRJBIeucI Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable Am Donnerstag, den 14.08.2014, 02:10 -0400 schrieb Rich Felker: > I think I have an informal proof sketch that this is necessary unless > we abandon requeue: > ... > With that in mind, I'd like to look for ways we can fix the bogus > waiter accounting for the mutex that seems to be the source of the bug > you found. One "obvious" (but maybe bad/wrong?) solution would be to > put the count on the mutex at the time of waiting (rather than moving > it there as part of broadcast), so that decrementing the mutex waiter > count is always the right thing to do in unwait. sounds like a good idea, at least for correctness > Of course this > possibly results in lots of spurious futex wakes to the mutex (every > time it's unlocked while there are waiters on the cv, which could be a > lot). I we'd be more careful in not spreading too much wakes where we shouldn't, there would perhaps not be "a lot" of such wakeups. > It would be nice if we had a separate field in the mutex (rather > than in the cv, as it is now) to store these on, and only move them to > the active waiters count at broadcast time, but I don't see any way to > get additional space in the mutex structure for this -- it's full. I thought of such designs, too, but one major problem (besides the space) with it is that a mutex can be used by several cv at a time. > > > 5. When can [timed]wait safely access the cv? > > >=20 > > > Only before unlocking the mutex, unless the implementation > > > synchronizes with possible signaling threads, or with destruction (an= d > > > possibly unmapping). Otherwise, per the above, it's possible that a > > > signaling thread destroys the cv. > >=20 > > so again this suggests an internal lock on the cv that would be used > > to synchronize between waiters and wakers? >=20 > This argument applies even to process-shared cv's, and for them, no > allocation is possible, at least difficult, for sure this would need support to allocate some object in the kernel and to use that object shared between processes :( > and I don't see a really good way to solve the > unmapping issue -- I think broadcast/signal would have to block > unmapping, and the last waiter to wake up would have to unblock it. > Maybe that's the right solution? probably, as I said, I don't have the right feeling for the mapping issues, yet. Jens --=20 :: INRIA Nancy Grand Est ::: AlGorille ::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: --=-wfvJQUxkvr7RRJBIeucI Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iEYEABECAAYFAlPsbIQACgkQD9PoadrVN+JDdACffDDQPOUbc7bS8Yt8QDlYffoi pzMAn2M1iEjT5TshPsHqArutABrpXYcA =msZV -----END PGP SIGNATURE----- --=-wfvJQUxkvr7RRJBIeucI--