Date: Mon, 02 Jun 2003 15:23:07 -0400 (EDT) From: John Baldwin <jhb@FreeBSD.org> To: Pawel Jakub Dawidek <nick@garage.freebsd.pl> Cc: freebsd-hackers@freebsd.org Subject: RE: Locking, locking... Message-ID: <XFMail.20030602152307.jhb@FreeBSD.org> In-Reply-To: <20030602075328.GV45118@garage.freebsd.pl>
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On 02-Jun-2003 Pawel Jakub Dawidek wrote:
> 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?
In the process case, the all process list is protected by an sx lock
(which you can hold across malloc(M_WAITOK)). Another solution in
some cases is that if the list grows, just copy out the first n items.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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