Re: [PATCH] xhci: use iopoll for xhci_handshake

From: Mathias Nyman
Date: Thu Feb 28 2019 - 03:53:45 EST

On 28.2.2019 9.09, Greg Kroah-Hartman wrote:
On Wed, Feb 27, 2019 at 03:19:17PM -0700, Daniel Kurtz wrote:
In cases such as xhci_abort_cmd_ring(), xhci_handshake() is called with
a spin lock held (and local interrupts disabled) with a huge 5 second
timeout. This can translates to 5 million calls to udelay(1). By its
very nature, udelay() is not meant to be precise, it only guarantees to
delay a minimum of 1 microsecond. Therefore the actual delay of
xhci_handshake() can be significantly longer. If the average udelay(1)
is greater than 2.2 us, the total time in xhci_handshake() - with
interrupts disabled can be > 11 seconds triggering the kernel's soft lockup

To avoid this, let's replace the open coded io polling loop with one from
iopoll.h that uses a loop timed with the more presumably reliable ktime

Signed-off-by: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>

Looks sane to me, nice fixup.

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Is this causing problems on older kernels/devices today such that we
should backport this?

A very similar patch was submitted some weeks ago by Andrey Smirnov.

His commit message only mentions that readl_poll_timeout_atomic() does the same job,
not about any issues with the loop, so I was going to send it forward to usb-next
after 5.1-rc (to 5.2).