tree ad66519a92b995920ecada788e4a08e265747545
parent 44672e4fbd40e2dda8bbce7d0f71d24dbfc7e00e
author Nick Piggin <npiggin@kernel.dk> 1282070253 +1000
committer Al Viro <viro@zeniv.linux.org.uk> 1282134946 -0400

fs: fs_struct rwlock to spinlock

fs: fs_struct rwlock to spinlock

struct fs_struct.lock is an rwlock with the read-side used to protect root and
pwd members while taking references to them. Taking a reference to a path
typically requires just 2 atomic ops, so the critical section is very small.
Parallel read-side operations would have cacheline contention on the lock, the
dentry, and the vfsmount cachelines, so the rwlock is unlikely to ever give a
real parallelism increase.

Replace it with a spinlock to avoid one or two atomic operations in typical
path lookup fastpath.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>