Hi
Browsing (and using) the excellent RCU infrastructure for files that was adopted for 2.6.14-rc1, I noticed that the file_lock spinlock sit close to mostly read fields of 'struct files_struct'
In SMP (and NUMA) environnements, each time a thread wants to open or close a file, it has to acquire the spinlock, thus invalidating the cache line containing this spinlock on other CPUS. So other threads doing read()/write()/... calls that use RCU to access the file table are going to ask further memory (possibly NUMA) transactions to read again this memory line.
Please consider applying this patch. It moves the spinlock to another cache line, so that concurrent threads can share the cache line containing 'count' and 'fdt' fields.