On Tue, Feb 20, 2018 at 09:39:37AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 20, 2018 at 12:34:57AM +0100, John Ogness wrote:
> > Implementation 2: Using switch on a dentry_lock_inode() that returns a
> > tristate value. Does not support branch prediction. This approach is
> > probably easiest to understand.
> >
> > /*
> > * Lock the inode. Might drop dentry->d_lock temporarily
> > * which allows inode to change. Start over if that happens.
> > */
> > switch (dentry_lock_inode(dentry)) {
> > case LOCK_FAST:
>
> Bah, I just checked, you cannot use GCC label attributes on statements
> :/ Otherwise you could've done:
>
> case LOCK_FAST: __attribute__((hot));
Oooh shiny, you can actually write:
switch(__builtin_expect(dentry_lock_inode(dentry), LOCK_FAST)) {
and have that work, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59521
> > break;
> > case LOCK_SLOW:
> > /*
> > * Recheck refcount as it might have been
> > * incremented while d_lock was dropped.
> > */
> > if (unlikely(dentry->d_lockref.count != 1))
> > goto drop_ref;
> > break;
> > case LOCK_FAILED:
> > goto again;
> > }
> >
