Re: [PATCH] usb: hcd: Cancel BH giveback works on removal

[Michal Pecio]

On Sun, 23 Aug 2026 10:31:28 -0400, Alan Stern wrote:
> On Sun, Aug 23, 2026 at 12:58:31PM +0200, Michal Pecio wrote:
> > Turns out, we do actually need to flush them, because workers use the
> > 'high_prio_bh' and 'low_prio_bh' members of 'usb_hcd' for a brief time
> > after all URBs are completed to track pending completions and possibly
> > reschedule themselves, see usb_giveback_urb_bh() implementation.
> > 
> > Flushing would suffice if the works don't reschedule themselves, but
> > cancel_work_sync() is more robust against stray completions.
> > 
> > Syzbot may have found the issue due to unlucky hard IRQ timing. It can
> > be reproduced by adding udelay(3000) in the work function, disabling RH
> > autosuspend to maintain the status URB and unbinding a real HC:
> > 
> > [10818.828029] ehci-pci 0000:00:12.0: USB bus 1 deregistered
> > [10818.828077] hcd_release freeing high_prio_bh ffff88814a950978
> > [10818.829211] usb_giveback_urb_bh still running on bh ffff88814a950978
> > 
> > Reported-by: syzbot+cade843a1e4af0651f5e@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/linux-usb/6a8a5047.dbb3a75c.13dd47.003e.GAE@xxxxxxxxxx/
> > Fixes: 94dfd7edfd5c ("USB: HCD: support giveback of URB in tasklet context")  
> 
> While that is logically correct, in fact the patch won't apply as-is to 
> any commit earlier than 8fea0c8fda30129b ("usb: core: hcd: Convert from 
> tasklet to BH workqueue").

Hmm, it's a fairly recent one, so this will only work on v6.12+.

Maybe not a big deal considering Greg's recent opinion about unbind
issues, though TBH I'm not really convinced about unbind's irrelevance
in this era where everything (including USB HCs) is hotpluggable.

I'm OK with this patch being dropped (I will maintain it for myself),
stripped of Cc:stable or limited to branches where it applies. And
I probably won't bother backporting it all they way to stone age.

Regards,
Michal