On Friday March 10, [email protected] wrote:
> On Fri, Mar 10, Neil Brown wrote:
>
> > -static void prune_dcache(int count)
> > +static void prune_dcache(int count, struct super_block *sb)
> > {
> > spin_lock(&dcache_lock);
> > for (; count ; count--) {
> > @@ -417,8 +425,10 @@ static void prune_dcache(int count)
> > spin_unlock(&dentry->d_lock);
> > continue;
> > }
> > - /* If the dentry was recently referenced, don't free it. */
> > - if (dentry->d_flags & DCACHE_REFERENCED) {
> > + /* If the dentry was recently referenced, or is for
> > + * a unmounting filesystem, don't free it. */
> > + if ((dentry->d_flags & DCACHE_REFERENCED) ||
> > + (dentry->d_sb != sb && dentry->d_sb->s_root == NULL)) {
> > dentry->d_flags &= ~DCACHE_REFERENCED;
> > list_add(&dentry->d_lru, &dentry_unused);
> > dentry_stat.nr_unused++;
>
> You have to down_read the rw-semaphore sb->s_umount since sb->s_root is
> protected by it :(
No you don't.
sb->s_root is set precisely twice for any filesystem.
Once when the filesystem is mounted, typically in the fill_super
function, and once in generic_shutdown_super where it is set to NULL.
There is no need to lock against the first change as the sb is not
globally visible until after s_root is set. So for the present
purpose we only need to worry about the second change.
For this, the current usage of dcache_lock is enough to remove any
possible race. generic_shutdown_super sets s_root to NULL and then
takes dcache_lock (via wait_for_prunes) before it cares if anyone has
noticed the NULL or not. prune_dcache holds dcache_lock while testing
for NULL. So there is no room for a race.
s_umount is sometimes taken before testing s_root. This is always
because the sb has just been found on the list of all superblocks, and
so the thread doesn't hold a proper reference to it. In our case
we are holding a dentry which in turn holds a real reference to the
superblock.
So I stand by my patch - s_root can be safely tested here.
NeilBrown
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