[PATCH] LFSR: Fix spin related bug

[PATCH] LFSR: Fix spin related bug

[Alex Pyrgiotis]

Hi Jens,

Thanks for the inclusion in your code, hopefully it was easy to review.
I stumbled upon a silly bug which may affect some rare cases, so you may
want to include that patch too, just in case.

Regards,
Alex

Alex Pyrgiotis (1):
  Fix spin related bug

 lib/lfsr.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

-- 
1.8.1.4

[PATCH] Fix spin related bug

[Alex Pyrgiotis]

On the previous patch on cycle detection, we would incremet spin value
when:

    num_vals % cycle_length == 0

which would evaluate always to true for the first iteration since we
start with num_vals = 0. Only one corner-case is affected by this bug
and this patch should fix it.

Signed-off-by: Alex Pyrgiotis <apyrgio@grnet.gr>

diff --git a/lib/lfsr.c b/lib/lfsr.c
index a835404..4c15c62 100644
--- a/lib/lfsr.c
+++ b/lib/lfsr.c
@@ -108,22 +108,28 @@ static inline void __lfsr_next(struct fio_lfsr *fl, unsigned int spin)
 	}
 }
 
+/*
+ * lfsr_next does the following:
+ *
+ * a. Return if the number of max values has been exceeded.
+ * b. Check if the next iteration(s) produce a cycle (due to spin) and add "1"
+ *    where necessary.
+ * c. Calculate the next value and return.
+ */
 int lfsr_next(struct fio_lfsr *fl, uint64_t *off, uint64_t last)
 {
 	int repeat;
 	unsigned int spin;
 
+	if (fl->num_vals++ > fl->max_val)
+		return 1;
+
 	repeat = fl->num_vals % fl->cycle_length;
 	if (repeat == 0)
 		spin = fl->spin + 1;
 	else
 		spin = fl->spin;
 
-	if (fl->num_vals > fl->max_val)
-		return 1;
-
-	fl->num_vals++;
-
 	do {
 		__lfsr_next(fl, spin);
 	} while (fl->last_val > fl->max_val);
-- 
1.8.1.4

Re: [PATCH] Fix spin related bug

[Jens Axboe]

On Sat, Mar 09 2013, Alex Pyrgiotis wrote:
> On the previous patch on cycle detection, we would incremet spin value
> when:
> 
>     num_vals % cycle_length == 0
> 
> which would evaluate always to true for the first iteration since we
> start with num_vals = 0. Only one corner-case is affected by this bug
> and this patch should fix it.

Thanks Alex, applied.

-- 
Jens Axboe