On Tue, Aug 25, 2020 at 11:20 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > Instead of holding the RCU read lock the whole time while accessing the > policy, add a simple refcount mechanism to track its lifetime. After > this, the RCU read lock is held only for a brief time when fetching the > policy pointer and incrementing the refcount. The policy struct is then > guaranteed to stay alive until the refcount is decremented. > > Freeing of the policy remains the responsibility of the task that does > the policy reload. In case the refcount drops to zero in a different > task, the policy load task is notified via a completion. That's an interesting pattern. Is this approach used anywhere else in the kernel? I didn't see any examples of it in the RCU documentation. > The advantage of this change is that the operations that access the > policy can now do sleeping allocations, since they don't need to hold > the RCU read lock anymore. This patch so far only leverages this in > security_read_policy() for the vmalloc_user() allocation (although this > function is always called under fsi->mutex and could just access the > policy pointer directly). The conversion of affected GFP_ATOMIC > allocations to GFP_KERNEL is left for a later patch, since auditing > which code paths may still need GFP_ATOMIC is not very easy. Technically we don't need this patch for that purpose because rcu_read_lock() isn't actually needed at all in security_read_policy(), so I think we're better off just getting rid of it there and letting it use rcu_dereference_check(..., 1) or rcu_dereference_protected() instead.
