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>

1 files changed, 5 insertions, 5 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 98b450876f93..856eac3ec52e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -752,13 +752,13 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
         struct fs_struct *fs = current->fs;
         if (clone_flags & CLONE_FS) {
                 /* tsk->fs is already what we want */
-                write_lock(&fs->lock);
+                spin_lock(&fs->lock);
                 if (fs->in_exec) {
-                        write_unlock(&fs->lock);
+                        spin_unlock(&fs->lock);
                         return -EAGAIN;
                 }
                 fs->users++;
-                write_unlock(&fs->lock);
+                spin_unlock(&fs->lock);
                 return 0;
         }
         tsk->fs = copy_fs_struct(fs);
@@ -1676,13 +1676,13 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
 
                 if (new_fs) {
                         fs = current->fs;
-                        write_lock(&fs->lock);
+                        spin_lock(&fs->lock);
                         current->fs = new_fs;
                         if (--fs->users)
                                 new_fs = NULL;
                         else
                                 new_fs = fs;
-                        write_unlock(&fs->lock);
+                        spin_unlock(&fs->lock);
                 }
 
                 if (new_mm) {