On Thu, Dec 02, 2021 at 08:59:52PM +0530, Charan Teja Kalla wrote: > > What part of the XArray documentation led you to believe that this is a > > safe thing to do? Because it needs to be rewritten immediately! > > The above code changes made from my understanding of both the > Documentation and the implementation of xa_for_each(). The Locking > section of the document[1] says that xa_for_each() takes the rcu lock > thus can be used without any explicit locking and the "Advanced API" > section says that users need to take xa_lock/rcu lock as no locking done > for you. > > Further I have looked at the xa_for_each() implementation details, > where, it is taking the rcu_lock just across xas_find() in both > xa_find() and xa_find_after() which made me to think that it just needs > to take the rcu lock just across the xas_find(). > > But a comment from you saying that this implementation is wrong making > me to think that I lack very trivial understanding about xarray usage. Would this change to the documentation have prevented you from making this mistake? The advanced API is based around the xa_state. This is an opaque data structure which you declare on the stack using the XA_STATE() macro. This macro initialises the xa_state ready to start walking around the XArray. It is used as a cursor to maintain the position in the XArray and let you compose various operations together without -having to restart from the top every time. +having to restart from the top every time. The contents of the xa_state +are protected by the rcu_read_lock() or the xas_lock(). If you need to +drop whichever of those locks is protecting your state and tree, you must +call xas_pause() so that future calls do not rely on the parts of the +state which were left unprotected.
