linux-kernel - Re: [PATCH] xhci: use iopoll for xhci

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Date:   Thu, 28 Feb 2019 10:55:44 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Kurtz <djkurtz@...omium.org>,
        Andrey Smirnov <andrew.smirnov@...il.com>
Cc:     rrangel@...omium.org, Mathias Nyman <mathias.nyman@...el.com>,
        "open list:USB XHCI DRIVER" <linux-usb@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xhci: use iopoll for xhci_handshake

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
>> detector.
>>
>> 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
>> infrastructure.
>>
>> Signed-off-by: Daniel Kurtz <djkurtz@...omium.org>
> 
> Looks sane to me, nice fixup.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> 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.

https://lore.kernel.org/lkml/20190208014816.21869-1-andrew.smirnov@gmail.com/

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).

-Mathias