Locking, locking...

[nick at garage.freebsd.pl (Pawel Jakub Dawidek)]

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_(_at_)_dawidek_(_dot_)_net
UNIX Systems Programmer/Administrator     http://garage.freebsd.pl
Am I Evil? Yes, I Am!                     http://cerber.sourceforge.net
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