Date: Fri, 27 Jul 2012 15:45:55 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Bruce Evans <brde@optusnet.com.au> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, Gleb Smirnoff <glebius@freebsd.org>, src-committers@freebsd.org Subject: Re: svn commit: r238828 - head/sys/sys Message-ID: <20120727124555.GE2676@deviant.kiev.zoral.com.ua> In-Reply-To: <20120727221529.K7360@besplex.bde.org> References: <201207270916.q6R9Gm23086648@svn.freebsd.org> <20120727111237.GC2676@deviant.kiev.zoral.com.ua> <20120727111904.GQ14135@FreeBSD.org> <20120727221529.K7360@besplex.bde.org>
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--mNseXQyS8ZYhN/HW Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Jul 27, 2012 at 10:32:55PM +1000, Bruce Evans wrote: > On Fri, 27 Jul 2012, Gleb Smirnoff wrote: >=20 > >On Fri, Jul 27, 2012 at 02:12:37PM +0300, Konstantin Belousov wrote: > >K> On Fri, Jul 27, 2012 at 09:16:48AM +0000, Gleb Smirnoff wrote: > >K> > ... > >K> > Log: > >K> > Add assertion for refcount overflow. > >K> > > >K> > Submitted by: Andrey Zonov <andrey zonov.org> > >K> > Reviewed by: kib > >K> It was discussed rather then reviewed. > >K> > >K> I suggest that the assert may be expressed as a check after the=20 > >increment, > >K> which verifies that counter is !=3D 0. This allows to avoid namespace > >K> pollution due to limits.h. > > > >Hmm, overflowing unsigned is a defined behavior in C. If Bruce agrees, > >then I'm happy with KASSERT after increment. >=20 > Comparing with (uint_t)-1 before is equivalent. You can even omit the > cast (depend on the default promotion). >=20 > I just noticed that there is a technical problem -- the count is read > unlocked in the KASSERT. And since the comparision is for equality, > if you lose the race reading the count when it reaches the overflow > threshold, then you won't see it overflow unless it wraps again and > you win the race next time (or later). atomic_cmpset could be used > to clamp the value at the max, but that is too much for an assertion. Yes, we discussed this with Gleb, and I do not see this as a problem. To make assert bullet-proof, either fetchadd() shall be used, as Gleb proposed, or some even more drastic measures applied. I did not liked fetchadd() proposal because it causes INVARIANTS code to use different function (and processor instruction in the end) comparing with !INVARIANTS case. >=20 > Simple locked reads of the count also don't prevent it wrapping and > going a bit higher than 0 with increments by other CPUs before the > CPU that notices the overflow can panic. So the patch in the PR may > have been better than the one committed (IIRC, it paniced some > time before wrapping, and people didn't like this). >=20 > I prefer to use signed types, even for, or especially for counters. > Then if the counter overflows you have a long time to notice this, > and may notice without explicit testing because negative counts are > printed somewhere. Integer overflow gives undefined behaviour > immediately, and there is a compiler flag to generate tests for it. > No one ever uses this, and it wouldn't work for variables that need > atomic accesses anyway. And there, people complain about loosing half of the counter capacity. --mNseXQyS8ZYhN/HW Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlASjYMACgkQC3+MBN1Mb4gI8gCeOAx9Ma/v2ElrE/OoueY0y+J2 hwEAoORRDL+TIXuO+8q/aDzvU0g/pzkX =ZC+z -----END PGP SIGNATURE----- --mNseXQyS8ZYhN/HW--
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