On Wed, Jan 12, 2011 at 2:59 PM, Ian Kent <raven@xxxxxxxxxx> wrote: > On Tue, 2011-01-11 at 11:57 -0600, Alex Elder wrote: >> On Tue, 2011-01-11 at 08:51 -0800, Linus Torvalds wrote: >> > On Tue, Jan 11, 2011 at 8:34 AM, Alex Elder <aelder@xxxxxxx> wrote: >> > > >> > > FYI, when using this code, as merged by Linus, I hit the >> > > BUG_ON() at the beginning of d_set_d_op() when it's called >> > > by autofs4_dir_mkdir(). I managed to work around it by >> > > just commenting out those BUG_ON() calls but it's something >> > > that ought to get addressed properly. >> > >> > Yeah, removing the BUG_ON() isn't the right thing to do - it means >> > that autofs4 is obviously setting the dentry ops twice for the same >> > dentry. >> > >> > Possibly the thing could be relaxed to allow setting the _same_ d_op >> > pointer, ie do something like >> > >> > if (dentry->d_op == op) >> > return; >> > >> > at the top of that function. But looking at it, I don't think that >> > fixes the autofs4 issue. >> >> That's easy enough, but it seems everybody else ensures >> this gets done just once per dentry, and it would be nice >> to preserve that "tightness" if possible. >> >> > The fact that autofs4 does "d_add()" before it sets the d_ops (or >> > other dentry state, for that matter) looks a bit scary. To me that >> > smells like it might get a dentry lookup hit before it's actually >> > fully done. >> >> Agreed. > > Isn't the parent i_mutex held during mkdir()? Yes but there are concurrency cases allowed without i_mutex. Lookup, for example, which ends up touching d_revalidate and when dropping the dentry, possibly d_delete. There seems no benefit to allowing switch of d_ops on a live dentry, and many downsides. So the rule should just be that it is not allowed. > Still the order can be changed, of course. > >> >> > Does it make any difference if you move the various d_add() calls down >> > to the end of the functions to when the "dentry" has really been >> > instantiated? >> >> Looking at it quickly, I don't think that would matter for >> the case at hand. I.e., that might be safer but it doesn't >> address the fact that these fields are getting initialized >> multiple times. > > Yeah, a hangover from changes done over time. > Not setting the dentry op in ->lookup() should fix this. How about negative dentries? They should be set up with d_ops upon allocation, preferably if your operations can handle negative dentries. Thanks, Nick -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
