Locking, locking...

[John Baldwin]

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 at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/