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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Date:   Mon, 24 Apr 2023 08:33:18 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     William Breathitt Gray <william.gray@...aro.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 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