Re: [PATCH mt76] wifi: mt76: mt792x: drop redundant napi_disable() in unregister path

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jul 28, 2026 at 05:20:48AM +0500, Mikhail Gavrilov wrote:
> Commit 13b7e6a96a00 ("wifi: mt76: Disable napi when removing device")
> made mt76_dma_cleanup() disable every RX NAPI instance before deleting
> it.  mt7921e_unregister_device() and mt7925e_unregister_device() already
> disable the very same instances and only afterwards call
> mt792x_dma_cleanup() -> mt76_dma_cleanup(), so each instance is now
> disabled twice.
> 
> napi_disable() is not idempotent: on return it leaves NAPIF_STATE_SCHED
> and NAPIF_STATE_NPSVC set, so a second call without an intervening
> napi_enable() spins in usleep_range() forever, waiting for bits that
> nobody will clear:
> 
>   task:modprobe        state:D stack:25720 pid:7954  tgid:7954
>   Call Trace:
>    <TASK>
>    __schedule+0x11b8/0x26d0
>    schedule+0xe7/0x2f0
>    schedule_hrtimeout_range_clock+0x218/0x330
>    usleep_range_state+0x133/0x1b0
>    napi_disable_locked+0x37d/0x5f0
>    napi_disable+0x43/0x80
>    mt76_dma_cleanup+0x2b4/0x860 [mt76]
>    mt7921_pci_remove+0x17f/0x350 [mt7921e]
>    pci_device_remove+0xb6/0x1e0
>    device_release_driver_internal+0x38d/0x540
>    driver_detach+0xd0/0x1b0
>    bus_remove_driver+0x127/0x2d0
>    pci_unregister_driver+0x2a/0x280
>    __do_sys_delete_module+0x36a/0x5b0
>    do_syscall_64+0x11c/0x6d0
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>    </TASK>
> 
> mt7921_pci_shutdown() and mt7925_pci_shutdown() reuse the remove path,
> so the same deadlock is hit on every reboot and poweroff.  It is silent:
> the stuck task keeps sleeping and rescheduling, so neither the hung task
> detector nor the lockup detectors fire, and the last line on the console
> is "systemd-shutdown[1]: Rebooting."
> 
> Drop the driver-side loops and rely on mt76_dma_cleanup() instead.
> mt792x_dma_cleanup() stops and resets the WFDMA engine before calling
> it, so RX is already quiesced when the NAPI instances are disabled
> there.
> 
> Fixes: 13b7e6a96a00 ("wifi: mt76: Disable napi when removing device")
> Reported-by: Bert Karwatzki <spasswolf@web.de>
> Closes: https://lore.kernel.org/all/20260724151419.26014-1-spasswolf@web.de/
> Suggested-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

While this patch fixes the shutdown hang for me too on a system using
mt7925e, Sashiko found that this patch introduces a use-after-free
because NAPI is now being disabled too late:
https://sashiko.dev/#/patchset/20260728002048.19351-1-mikhail.v.gavrilov%40gmail.com
Should 13b7e6a96a00 be reverted, then fixed in another way such as
calling napi_disable() in mt7915_unregister_device()?

- Eric