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>
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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 = 0;
SLIST_FOREACH(elem, &list_head, l_next) {
n++;
}
mtx_unlock(&mtx_list_lock);
tab = 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!
--
Pawel Jakub Dawidek pawel@dawidek.net
UNIX Systems Programmer/Administrator http://garage.freebsd.pl
Am I Evil? Yes, I Am! http://cerber.sourceforge.net
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