On 25/05/16 21:03, Paolo Bonzini wrote: >> The page table seems to be protected by 'mmap_lock' in user mode >> emulation but by 'tb_lock' in system mode emulation. It may turn to be >> possible to read it safely even with no lock held. > Yes, it is possible to at least follow the radix tree safely with no > lock held. The fields in the leaves can be either lockless or protected > by a lock. > > The radix tree can be followed without a lock just like you do with RCU. > The difference with RCU is that: > > 1) the leaves are protected with a lock, so you don't do the "copy"; > instead after reading you lock around updates > > 2) the radix tree is only ever added to, so you don't need to protect > the reads with rcu_read_lock/rcu_read_unlock. rcu_read_lock and > rcu_read_unlock are only needed to inform the deleters that something > cannot yet go away. Without deleters, you don't need rcu_read_lock > and rcu_read_unlock (but you still need atomic_rcu_read/atomic_rcu_set). > >
Yes, however looking closer at how the leafs are used I can't see much point to do this so far... Thanks, Sergey
