On Mon, 28 Nov 2005, William H. Taber wrote:
> Ian Kent wrote:
>
> > My thoughts:
> >
> > The cause of this issue is user space programs using autofs4 need to
> > call services that must be able to take the inode semaphore. Notably
> > sys_mkdir and sys_symlink in order to complete their task.
> >
> > I believe that, in this case, releasing the semaphore is ok since the
> > entry is part of the autofs filesystem and so autofs is responsible for
> > taking care of it, provided that it is done carefully. The semaphore is
> > meant to serialize changes being to the directory and these changes are
> > done in autofs by asking the user space process to do it. Which are
> > themselves serialized by the same semaphore.
> >
> > The only tricky thing I can think of here is that care must be taken to
> > ensure that the semaphore is not released before the DCACHE_AUTOFS_PENDING
> > flag is set to make sure that other incoming requests are sent to the wait
> > queue.
> >
> > The attached patch does this and opts for a conservative approach by
> > broadening the critical region instead of narrowing it.
> >
> > It may also be necessary to review the return codes from revaliate but I'm
> > only part way through that.
> >
> > Please review and test this patch and offer further comment.
> > Sorry guys but I haven't been able to test this at all save verifying that
> > it compiles.
> >
> > Hopefully I haven't missed anything completely obvious ... DOH!
> >
> > Ian
> >
> > --- linux-2.6.15-rc1/fs/autofs4/root.c.lookup-deadlock 2005-11-17
> > 18:58:38.000000000 +0800
> > +++ linux-2.6.15-rc1/fs/autofs4/root.c 2005-11-27 17:00:40.000000000
> > +0800
> > @@ -487,11 +487,8 @@ static struct dentry *autofs4_lookup(str
> > dentry->d_fsdata = NULL;
> > d_add(dentry, NULL);
> >
> > - if (dentry->d_op && dentry->d_op->d_revalidate) {
> > - up(&dir->i_sem);
> > + if (dentry->d_op && dentry->d_op->d_revalidate)
> > (dentry->d_op->d_revalidate)(dentry, nd);
> > - down(&dir->i_sem);
> > - }
> >
> > /*
> > * If we are still pending, check if we had to handle
> > --- linux-2.6.15-rc1/fs/autofs4/waitq.c.lookup-deadlock 2005-11-27
> > 17:09:42.000000000 +0800
> > +++ linux-2.6.15-rc1/fs/autofs4/waitq.c 2005-11-27 17:17:34.000000000
> > +0800
> > @@ -161,6 +161,8 @@ int autofs4_wait(struct autofs_sb_info *
> > enum autofs_notify notify)
> > {
> > struct autofs_wait_queue *wq;
> > + struct inode *dir = dentry->d_parent->d_inode;
> > + int i_sem_held;
> > char *name;
> > int len, status;
> >
> > @@ -227,6 +229,14 @@ int autofs4_wait(struct autofs_sb_info *
> > (unsigned long) wq->wait_queue_token, wq->len,
> > wq->name, notify);
> > }
> >
> > + /*
> > + * If we are called from lookup or lookup_hash the
> > + * the inode semaphore needs to be released for
> > + * userspace to do its thing.
> > + */
> > + i_sem_held = down_trylock(&dir->i_sem);
> > + up(&dir->i_sem);
> > +
> > if (notify != NFY_NONE && atomic_dec_and_test(&wq->notified)) {
> > int type = (notify == NFY_MOUNT ?
> > autofs_ptype_missing : autofs_ptype_expire_multi);
> > @@ -268,6 +278,10 @@ int autofs4_wait(struct autofs_sb_info *
> > DPRINTK("skipped sleeping");
> > }
> >
> > + /* Re-take the inode semaphore if it was held */
> > + if (i_sem_held)
> > + down(&dir->i_sem);
> > +
> > status = wq->status;
> >
> > /* Are we the last process to need status? */
> > -
> Ian,
> I have not tested this patch but it seems to have a serious flaw. Given
> that do_lookup does not get the parent i_sem lock before calling
> revalidate, you have the possibility that you are being called without
> having gotten the lock but the lock may be held by another process. In
> that case you do not want to be releasing their lock while they are
> relying on it.
Oops.
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