Messages in this thread | | | From | Neil Brown <> | Date | Mon, 13 Mar 2006 08:57:09 +1100 | Subject | Re: [PATCH] Fix shrink_dcache_parent() against shrink_dcache_memory() race (3rd updated patch)] |
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On Friday March 10, jblunck@suse.de 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.
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