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Date:   Thu, 24 Jan 2019 20:04:05 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Will Deacon <will.deacon@arm.com>
Cc:     linux-kernel@vger.kernel.org, Dave Airlie <airlied@redhat.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH stable-4.9.y] locking/qspinlock: Pull in asm/byteorder.h
 to ensure correct endianness
Message-ID: <20190124190405.GB32078@kroah.com>
References: <20190124185415.29830-1-will.deacon@arm.com>
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On Thu, Jan 24, 2019 at 06:54:15PM +0000, Will Deacon wrote:
> From: Dave Airlie <airlied@redhat.com>
> 
> This commit is not required upstream, but is required for the 4.9.y
> stable series.
> 
> Upstream commit 101110f6271c ("Kbuild: always define endianess in
> kconfig.h") ensures that either __LITTLE_ENDIAN or __BIG_ENDIAN is
> defined to reflect the endianness of the target CPU architecture
> regardless of whether or not <asm/byteorder.h> has been #included. The
> upstream definition of 'struct qspinlock' relies on this property.
> 
> Unfortunately, the 4.9.y stable series does not provide this guarantee,
> so the 'spin_unlock()' routine can erroneously treat the underlying
> lockword as big-endian on little-endian architectures using native
> qspinlock (i.e. x86_64 without PV) if the caller has not included
> <asm/byteorder.h>. This can lead to hangs such as the one in
> 'i915_gem_request()' reported via bugzilla:
> 
>   https://bugzilla.kernel.org/show_bug.cgi?id=202063
> 
> Fix the issue by ensuring that <asm/byteorder.h> is #included in
> <asm/qspinlock_types.h>, where 'struct qspinlock' is defined.

That is crazy...

Thanks for the patch, now queued up.

greg k-h