On Wed, Jul 06, 2022 at 09:36:42AM +0300, Nikolay Borisov wrote:
> On 5.07.22 г. 23:29 ч., Matthew Wilcox wrote:
> > On Tue, Jul 05, 2022 at 09:26:47PM +0100, Filipe Manana wrote:
> > > In this case we can actually call xa_insert() without holding that
> > > spinlock, it's safe against other concurrent calls to
> > > btrfs_get_or_create_delayed_node(), btrfs_get_delayed_node(),
> > > btrfs_kill_delayed_inode_items(), etc.
> > >
> > > However, looking at xa_insert() we have:
> > >
> > > xa_lock(xa);
> > > err = __xa_insert(xa, index, entry, gfp);
> > > xa_unlock(xa);
> > >
> > > And xa_lock() is defined as:
> > >
> > > #define xa_lock(xa) spin_lock(&(xa)->xa_lock)
> > >
> > > So we'll always be under a spinlock even if we change btrfs to not
> > > take the root->inode_lock spinlock.
> > >
> > > This seems more like a general problem outside btrfs' control.
> > > So CC'ing Willy to double check.
> >
> > No, the XArray knows about its own spinlock. It'll drop it if it needs
> > to allocate memory and the GFP flags indicate that the caller can sleep.
> > It doesn't know about your spinlock, so it can't do the same thing for
> > you ;-)
>
>
> In order to catch (and prevent) further offensive can we perhaps have
> something like that in xa_insert:
>
>
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index c29e11b2c073..63c00b2945a2 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -770,6 +770,9 @@ static inline int __must_check xa_insert(struct xarray
> *xa,
> {
> int err;
>
> + if (gfpflags_allow_blocking(gfp))
> + might_sleep()
> +
> xa_lock(xa);
> err = __xa_insert(xa, index, entry, gfp);
> xa_unlock(xa);
I think you mean:
might_alloc(gfp);
And yes, I think that makes a lot of sense. Quite a few similar places
to do ... I'll take care of it.
