Date: Mon, 2 Jun 2003 09:53:28 +0200 From: Pawel Jakub Dawidek <nick@garage.freebsd.pl> To: freebsd-hackers@freebsd.org Subject: Locking, locking... Message-ID: <20030602075328.GV45118@garage.freebsd.pl>
next in thread | raw e-mail | index | archive | help
--aPbDRKcb5+hBUoJi
Content-Type: text/plain; charset=iso-8859-2
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable
Hello hackers...
I need advice how to handle with one locking problem.
For example I got some list with some values. This list could be
modified by any process. I want to get values from this list and
copy them to allocated table. But how to count number of needed
fields?
mtx_lock(&mtx_list_lock);
n =3D 0;
SLIST_FOREACH(elem, &list_head, l_next) {
n++;
}
mtx_unlock(&mtx_list_lock);
tab =3D malloc(n * sizeof(something), M_TEMP, M_WAITOK);
mtx_lock(&mtx_list_lock);
[...]
As we all knew size of list could be changed when we were in malloc().
Of course we could check list size again after malloc() and mtx_lock(),
but what to do when it was changed? Recall memory allocation?
If size of this list depends on every process there is a chance to DoS
such piece of code. Return an error? Not always it is possible.
IMHO there should be malloc() version that could be called under lock.
Malloc() M_NOWAIT could be called in such scenario?
How do you handle with situations like this?
Thanks!
--=20
Pawel Jakub Dawidek pawel@dawidek.net
UNIX Systems Programmer/Administrator http://garage.freebsd.pl
Am I Evil? Yes, I Am! http://cerber.sourceforge.net
--aPbDRKcb5+hBUoJi
Content-Type: application/pgp-signature
Content-Disposition: inline
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.7 (FreeBSD)
iQCVAwUBPtsCeD/PhmMH/Mf1AQEilwP+N/Umn+2e4+VFSQisBa6hKpIXfyPFFvoY
xnBoJuzBp+clBVn7cgTPHy1TfhWZF4caNcFBcjQLHENvdI/G45pFyXP3Xoe3BSrO
bXSTxevhzz7ISBxmjmh20acI3RNY3BdRNuKuXNN4Kob5TdMb1Lp72t3O/txEQCDf
DweTa/Buv1g=
=RcVv
-----END PGP SIGNATURE-----
--aPbDRKcb5+hBUoJi--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20030602075328.GV45118>