On Mon, Apr 21, 2008 at 10:30:19AM -0700, Linus Torvalds wrote: > > > On Mon, 21 Apr 2008, Paul E. McKenney wrote: > > > > I did take a quick look for improperly freeing dentries -- unhashed > > dentries are freed directly, so if there is a code path that somehow > > unhashes dentries and then d_free()s them without a grace period, we > > have a problem. > > No, not even then. > > We *always* unhash the dentries before freeing them, but we very > consciously use "hlist_del_rcu()" on them, not "hlist_del_init()". > > That, in turn, will mean that the "pprev" pointer will still be set, so > the "hlist_unhashed()" thing will *not* trigger. > > IOW, when we do that direct-free with: > > if (hlist_unhashed(&dentry->d_hash)) > __d_free(dentry); > > the "hlist_unhashed()" will literally guarantee that i has *never* been on > a hash-list at all! Got it, hlist_del_rcu() sets ->pprev to LIST_POISON2, which is non-NULL, so the dentry still gets to wait for a grace period. Color me blind!!! > (If you want to test whether it is currently unhashed or not, you actually > have to use "d_unhashed()" on the dentry under the dentry lock, which > tests the DCACHE_UNHASHED bit). And as it looks like you guessed, I was misreading the hlist_unhashed() above as d_unhashed(). :-/ Thanx, Paul > Of course, there could be some bug in there, but the thing is, none of > this has even changed in a long time, certainly not since 2.6.25. Which is > why I think the dcache code is all fine, and the bug comes from somewhere > else corrupting the data structures. > > Linus -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
