Re: [RFC 7/7] net: don't check for active hrtimer after adding it

From: Chris Redpath
Date: Wed Jul 09 2014 - 06:33:09 EST

Hi Viresh,

On 09/07/14 07:55, Viresh Kumar wrote:
hrtimer_start*() family never fails to enqueue a hrtimer to a clock-base. The
only special case is when the hrtimer was in past. If it is getting enqueued to
local CPUs's clock-base, we raise a softirq and exit, else we handle that on
next interrupt on remote CPU.

At several places in the kernel, we try to make sure if hrtimer was added
properly or not by calling hrtimer_active(), like:

hrtimer_start(timer, expires, mode);
if (hrtimer_active(timer)) {
/* Added successfully */
} else {
/* Was added in the past */

As hrtimer_start*() never fails, hrtimer_active() is guaranteed to return '1'.
So, there is no point calling hrtimer_active().

This patch updates net core to get this fixed.

Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
net/core/pktgen.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index fc17a9d..f911acd 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2186,8 +2186,6 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
do {
hrtimer_start_expires(&t.timer, HRTIMER_MODE_ABS);
- if (!hrtimer_active(&t.timer))
- t.task = NULL;

if (likely(t.task))

I think this if condition can also be removed. hrtimer_init_sleeper copies the supplied task_struct * to the timer, which in this case is 'current'. The check is likely to be there in case of !active case you removed.

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