releases/5.1.12/ras-cec-fix-binary-search-function.patch

   1 From f3c74b38a55aefe1004200d15a83f109b510068c Mon Sep 17 00:00:00 2001
   2 From: Borislav Petkov <bp@suse.de>
   3 Date: Sat, 20 Apr 2019 13:27:51 +0200
   4 Subject: RAS/CEC: Fix binary search function
   5
   6 From: Borislav Petkov <bp@suse.de>
   7
   8 commit f3c74b38a55aefe1004200d15a83f109b510068c upstream.
   9
  10 Switch to using Donald Knuth's binary search algorithm (The Art of
  11 Computer Programming, vol. 3, section 6.2.1). This should've been done
  12 from the very beginning but the author must've been smoking something
  13 very potent at the time.
  14
  15 The problem with the current one was that it would return the wrong
  16 element index in certain situations:
  17
  18   https://lkml.kernel.org/r/CAM_iQpVd02zkVJ846cj-Fg1yUNuz6tY5q1Vpj4LrXmE06dPYYg@mail.gmail.com
  19
  20 and the noodling code after the loop was fishy at best.
  21
  22 So switch to using Knuth's binary search. The final result is much
  23 cleaner and straightforward.
  24
  25 Fixes: 011d82611172 ("RAS: Add a Corrected Errors Collector")
  26 Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
  27 Signed-off-by: Borislav Petkov <bp@suse.de>
  28 Cc: Tony Luck <tony.luck@intel.com>
  29 Cc: linux-edac <linux-edac@vger.kernel.org>
  30 Cc: <stable@vger.kernel.org>
  31 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  32
  33 ---
  34  drivers/ras/cec.c |   34 ++++++++++++++++++++--------------
  35  1 file changed, 20 insertions(+), 14 deletions(-)
  36
  37 --- a/drivers/ras/cec.c
  38 +++ b/drivers/ras/cec.c
  39 @@ -181,32 +181,38 @@ static void cec_work_fn(struct work_stru
  40   */
  41  static int __find_elem(struct ce_array *ca, u64 pfn, unsigned int *to)
  42  {
  43 +       int min = 0, max = ca->n - 1;
  44         u64 this_pfn;
  45 -       int min = 0, max = ca->n;
  46
  47 -       while (min < max) {
  48 -               int tmp = (max + min) >> 1;
  49 +       while (min <= max) {
  50 +               int i = (min + max) >> 1;
  51
  52 -               this_pfn = PFN(ca->array[tmp]);
  53 +               this_pfn = PFN(ca->array[i]);
  54
  55                 if (this_pfn < pfn)
  56 -                       min = tmp + 1;
  57 +                       min = i + 1;
  58                 else if (this_pfn > pfn)
  59 -                       max = tmp;
  60 -               else {
  61 -                       min = tmp;
  62 -                       break;
  63 +                       max = i - 1;
  64 +               else if (this_pfn == pfn) {
  65 +                       if (to)
  66 +                               *to = i;
  67 +
  68 +                       return i;
  69                 }
  70         }
  71
  72 +       /*
  73 +        * When the loop terminates without finding @pfn, min has the index of
  74 +        * the element slot where the new @pfn should be inserted. The loop
  75 +        * terminates when min > max, which means the min index points to the
  76 +        * bigger element while the max index to the smaller element, in-between
  77 +        * which the new @pfn belongs to.
  78 +        *
  79 +        * For more details, see exercise 1, Section 6.2.1 in TAOCP, vol. 3.
  80 +        */
  81         if (to)
  82                 *to = min;
  83
  84 -       this_pfn = PFN(ca->array[min]);
  85 -
  86 -       if (this_pfn == pfn)
  87 -               return min;
  88 -
  89         return -ENOKEY;
  90  }
  91