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SubjectRe: [syzbot] WARNING: refcount bug in __linkwatch_run_queue
On Wed, Nov 17, 2021 at 06:15:48AM -0800, Jakub Kicinski wrote:
> On Wed, 17 Nov 2021 09:19:07 +0100 Willy Tarreau wrote:
> > Thanks for the report. I'm seeing that linkwatch_do_dev() is also
> > called in linkwatch_forget_dev(), and am wondering if we're not
> > seeing a sequence like this one:
> >
> > linkwatch_forget_dev()
> > list_del_init()
> > linkwatch_do_dev()
> > netdev_state_change()
> > ... one of the notifiers
> > ... linkwatch_add_event() => adds to watch list
> > dev_put()
> > ...
> >
> > __linkwatch_run_queue()
> > linkwatch_do_dev()
> > dev_put()
> > => bang!
> >
> > Well, in theory, no, since linkwatch_add_event() will call dev_hold()
> > when adding to the list, so we ought to leave the first call with a
> > refcount still covering the list's presence, and I don't see how it
> > can reach zero before reaching dev_put() in linkwatch_do_dev() as this
> > function is only called when the event was picked from the list.
> >
> > The only difference I'm seeing is that before the patch, a call to
> > linkwatch_forget_dev() on a non-present device would call dev_put()
> > without going through dev_activate(), dev_deactivate(), nor
> > netdev_state_change(), but I'm not seeing how that could make a
> > difference. linkwatch_forget_dev() is called from netdev_wait_allrefs()
> > which will wait for the refcnt to be exactly 1, thus even if we queue
> > an extra event we cant leave that function until the event has been
> > processed.
>
> The ref leak could come from anywhere, tho. Like:
>
> https://lore.kernel.org/all/87a6i3t2zg.fsf@nvidia.com/

OK thanks for the link, so better wait for this part to clarify itself
and see if the issue magically disappears ?

Willy

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