linux-kernel - Re: [PATCH 5.10 v4 4/5] counter: 104-quad-8: Fix race condition between FLAG and CNTR reads

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Message-ID: <ZEajjyUDN9z8FXYP@fedora>
Date:   Mon, 24 Apr 2023 11:43:11 -0400
From:   William Breathitt Gray <william.gray@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jonathan Cameron <jic23@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 5.10 v4 4/5] counter: 104-quad-8: Fix race condition
 between FLAG and CNTR reads

On Mon, Apr 24, 2023 at 08:33:18AM +0200, Greg Kroah-Hartman wrote:
> On Sun, Apr 23, 2023 at 07:20:46PM -0400, William Breathitt Gray wrote:
> > commit 4aa3b75c74603c3374877d5fd18ad9cc3a9a62ed upstream.
> > 
> > The Counter (CNTR) register is 24 bits wide, but we can have an
> > effective 25-bit count value by setting bit 24 to the XOR of the Borrow
> > flag and Carry flag. The flags can be read from the FLAG register, but a
> > race condition exists: the Borrow flag and Carry flag are instantaneous
> > and could change by the time the count value is read from the CNTR
> > register.
> > 
> > Since the race condition could result in an incorrect 25-bit count
> > value, remove support for 25-bit count values from this driver.
> > 
> > Fixes: 28e5d3bb0325 ("iio: 104-quad-8: Add IIO support for the ACCES 104-QUAD-8")
> > Cc: <stable@...r.kernel.org> # 5.10.x
> > Signed-off-by: William Breathitt Gray <william.gray@...aro.org>
> > ---
> >  drivers/counter/104-quad-8.c | 18 +++---------------
> >  1 file changed, 3 insertions(+), 15 deletions(-)
> 
> This patch breaks the build:
> 
>   CC [M]  drivers/counter/104-quad-8.o
> drivers/counter/104-quad-8.c: In function ‘quad8_read_raw’:
> drivers/counter/104-quad-8.c:117:34: error: ‘QUAD8_FLAG_BT’ undeclared (first use in this function); did you mean ‘QUAD8_FLAG_E’?
>   117 |                 borrow = flags & QUAD8_FLAG_BT;
>       |                                  ^~~~~~~~~~~~~
>       |                                  QUAD8_FLAG_E
> drivers/counter/104-quad-8.c:117:34: note: each undeclared identifier is reported only once for each function it appears in
> drivers/counter/104-quad-8.c:118:36: error: ‘QUAD8_FLAG_CT’ undeclared (first use in this function); did you mean ‘QUAD8_FLAG_E’?
>   118 |                 carry = !!(flags & QUAD8_FLAG_CT);
>       |                                    ^~~~~~~~~~~~~
>       |                                    QUAD8_FLAG_E
> 
> 

Ah, the 5.4 and 5.10 patches are missing the IIO changes from the 4.19
patch. I'll adjust and resubmit these two patches.

William Breathitt Gray

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