From 2da7d3ba367aecc0beeafd0a609393607a7f791e Mon Sep 17 00:00:00 2001 From: Borislav Petkov Date: Sat, 20 Apr 2019 13:27:51 +0200 Subject: [PATCH] RAS/CEC: Fix binary search function commit f3c74b38a55aefe1004200d15a83f109b510068c upstream. Switch to using Donald Knuth's binary search algorithm (The Art of Computer Programming, vol. 3, section 6.2.1). This should've been done from the very beginning but the author must've been smoking something very potent at the time. The problem with the current one was that it would return the wrong element index in certain situations: https://lkml.kernel.org/r/CAM_iQpVd02zkVJ846cj-Fg1yUNuz6tY5q1Vpj4LrXmE06dPYYg@mail.gmail.com and the noodling code after the loop was fishy at best. So switch to using Knuth's binary search. The final result is much cleaner and straightforward. Fixes: 011d82611172 ("RAS: Add a Corrected Errors Collector") Reported-by: Cong Wang Signed-off-by: Borislav Petkov Cc: Tony Luck Cc: linux-edac Cc: Signed-off-by: Greg Kroah-Hartman --- drivers/ras/cec.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) diff --git a/drivers/ras/cec.c b/drivers/ras/cec.c index adbf1a9e089e..c7205cdcc11a 100644 --- a/drivers/ras/cec.c +++ b/drivers/ras/cec.c @@ -185,32 +185,38 @@ static void cec_timer_fn(unsigned long data) */ static int __find_elem(struct ce_array *ca, u64 pfn, unsigned int *to) { + int min = 0, max = ca->n - 1; u64 this_pfn; - int min = 0, max = ca->n; - while (min < max) { - int tmp = (max + min) >> 1; + while (min <= max) { + int i = (min + max) >> 1; - this_pfn = PFN(ca->array[tmp]); + this_pfn = PFN(ca->array[i]); if (this_pfn < pfn) - min = tmp + 1; + min = i + 1; else if (this_pfn > pfn) - max = tmp; - else { - min = tmp; - break; + max = i - 1; + else if (this_pfn == pfn) { + if (to) + *to = i; + + return i; } } + /* + * When the loop terminates without finding @pfn, min has the index of + * the element slot where the new @pfn should be inserted. The loop + * terminates when min > max, which means the min index points to the + * bigger element while the max index to the smaller element, in-between + * which the new @pfn belongs to. + * + * For more details, see exercise 1, Section 6.2.1 in TAOCP, vol. 3. + */ if (to) *to = min; - this_pfn = PFN(ca->array[min]); - - if (this_pfn == pfn) - return min; - return -ENOKEY; } -- 2.20.1