Bug #236

Updated by Sven Eckelmann over 1 year ago

Simon debugged the refcnt problem and submitted some patches to fix them. I had a brief look and noticed that there are possible more problems similar to the <code>*list_del*</code> ones - just with <code>*list_add*</code>. Basically some functions use some kind of get function, notice that the element does not exist and then create a new one to add to the list. Only the "<code>list_add</code>" is protected. The result may be that an element in twice in a list when only a single occurrence is allowed.

The problem I saw is that functions adding objects in an RCU protected list are missing an definitive check. They first call some kind of <code>*_get</code> (<code>rcu_read_lock</code> only) to check if an object with this value already exists and then uses some kind of <code>*_add</code> to allocate a new object and add it (which may already be added in by a different context). So it has to be made sure that nothing modifies the list between the check and the add of the new object).

An RFC was submitted to demonstrate how it could be fixed