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>
diff --git a/kernel/fork.c b/kernel/fork.c
index 98b4508..856eac3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -752,13 +752,13 @@
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 @@
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) {