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>

fs/exec.c

@@ -1117,7 +1117,7 @@ int check_unsafe_exec(struct linux_binprm *bprm)
 	bprm->unsafe = tracehook_unsafe_exec(p);
 
 	n_fs = 1;
-	write_lock(&p->fs->lock);
+	spin_lock(&p->fs->lock);
 	rcu_read_lock();
 	for (t = next_thread(p); t != p; t = next_thread(t)) {
 		if (t->fs == p->fs)
@@ -1134,7 +1134,7 @@ int check_unsafe_exec(struct linux_binprm *bprm)
 			res = 1;
 		}
 	}
-	write_unlock(&p->fs->lock);
+	spin_unlock(&p->fs->lock);
 
 	return res;
 }