Re: [PATCH net] net: usbnet: fix softirq storm on suspend

On Mon, Sep 3, 2012 at 4:26 PM, Bjørn Mork [off-list ref] wrote:

Suspending an open usbnet device results in constant
rescheduling of usbnet_bh.

commit 65841fd5 "usbnet: handle remote wakeup asap"
refactored the usbnet_bh code to allow sharing the
urb allocate and submit code with usbnet_resume. In
this process, a test for, and immediate return on,
ENOLINK from rx_submit was unintentionally dropped.

The rx queue will not grow if rx_submit fails,
making usbnet_bh reschedule itself.  This results
in a softirq storm if the error is persistent.
rx_submit translates the usb_submit_urb error
EHOSTUNREACH into ENOLINK, so this is an expected
and persistent error for a suspended device. The
old code tested for this condition and avoided
rescheduling.  Putting this test back.

Cc: <redacted> # v3.5
Cc: Ming Lei <redacted>
Cc: Oliver Neukum <redacted>
Signed-off-by: Bjørn Mork <redacted>
---
Sorry for not noticing this before, but commit 65841fd5
makes usbnet autosuspend completely unusable.  The device
is suspended fine, but burning one CPU core at full load
uses a tiny bit more power making the power saving
negative...

I am wondering how you can reproduce the issue.

usbnet_terminate_urbs is called inside usbnet_suspend to
consume all URBs and SKBs, and rx_alloc_submit can't be
called during the period because of !netif_device_present().
Once usbnet_terminate_urbs and netif_device_attach() are
completed, who will schedule tasklet to trigger rx_alloc_submit?

I hope this can go into 3.6 and 3.5-stable ASAP. It is
a hard to notice regression, but all the same a serious
one.

Thanks,
--
Ming Lei
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