Bit spinlocks are problematic if PREEMPT_RT is enabled. They disable preemption, which is undesired for latency reasons and breaks when regular spinlocks are taken within the bit_spinlock locked region because regular spinlocks are converted to 'sleeping spinlocks' on RT. Bit spinlocks are also not covered by lock debugging, e.g. lockdep. With the spinlock substitution in place, they can be exposed via a new config switch: CONFIG_DEBUG_BIT_SPINLOCKS. This series handles only buffer head and jbd2, but does not touch the hlist_bl bit-spinlock usage. For most usage sites of hlist_bl the protected sections are either really small and do not contain any RT incompatible code or can be trivially fixed up. In those cases converting the list_bl locking to spinlocks looks like overkill, but it still might make sense to provide it for debugging. The only exception for this is the code in drivers/md/dm-snap.c which has interesting nested locking in place (via kmemcache, cgroups, block ...) which completely escapes lockdep. I pondered making the hlist_bl locking conditional on a per source file define, but that does not work due to include hell. Thoughts? Thanks, tglx
