Hi Ikegami,
On Fri, 1 Mar 2019 23:51:16 +0900
"Tokunori Ikegami" <ikegami...@yahoo.co.jp> wrote:
> > Except this version no longer does what Vignesh suggested. See how you
> > no longer test if chip_good() is true if time_after() returns true. So,
> > imagine the thread entering this function is preempted just after the
> > first chip_good() test, and resumed a few ms later. time_after() will
> > return true, but chip_good() might also return true, and you ignore it.
>
> I think that the following 3 versions will be worked for time_after() as a
> same result and follow the Vignesh-san suggestion.
Let me show you how they are different:
>
> 1. Original Vignesh-san suggestion
>
> if (chip_good(map, adr, datum)) {
> xip_enable(map, chip, adr);
> goto op_done;
> }
--> thread preempted here
==> chip_good() test becomes valid here
--> thread resumed here, but timeout has expired
>
> if (time_after(jiffies, timeo)) {
you enter this branch
> /* Test chip_good() if TRUE incorrectly again so write failure
> by time_after() can be avoided. */
> if (chip_good(map, adr, datum)) {
chip_good() returns true
> xip_enable(map, chip, adr);
> goto op_done;
> }
> break;
> }
>
> 2. Liujian-san v3 patch
>
> /* Test chip_good() if FALSE correctly so write failure by time_after()
> can be avoided. */
--> thread preempted here
==> chip_good() test becomes valid here
--> thread resumed here, but timeout has expired
> if (time_after(jiffies, timeo) && !chip_good(map, adr))
You do not enter this branch because the chip_good() test is done once
more in case of timeout.
> break;
>
> if (chip_good(map, adr, datum)) {
> xip_enable(map, chip, adr);
> goto op_done;
> }
>
> 3. My idea
>
> /* Save current jiffies value before chip_good() to avoid write failure
> by time_after() without testing chip_good() again. */
> unsigned long now = jiffies;
>
> if (chip_good(map, adr, datum)) {
> xip_enable(map, chip, adr);
> goto op_done;
> }
>
--> thread preempted here
==> chip_good() test becomes valid here
--> thread resumed here, but timeout has expired
> if (time_after(now, timeo))
You do enter this branch, and erroneously report a failure.
> break;
>
See now why your version is not correct?
Regards,
Boris
