Thanks for highlighting this issue. Typically, the expired timer callback sets
the timer_id to invalid which prevents confusion.
I'll pass along the SCTP report.
Dave
-----Original Message-----
From: vpp-dev@lists.fd.io <vpp-dev@lists.fd.io> On Behalf Of Andreas Schultz
Sent: Thursday, July 4, 2019 8:21 AM
To: vpp-dev@lists.fd.io
Subject: [vpp-dev] unsave tw_timer_expire + tw_timer_stop + tw_timer_start
pattern
Hi,
I've stumble upon a problem with a specific sequence of expiring, stopping and
(re)starting tw timers.
In pseudo C code the pattern is
Lets assume some kind of timer in a session gets started with:
session->timer[0] = tw_timer_start (tw, interval_a); timer[1] =
session->tw_timer_start (tw, interval_b);
...
session->timer[n] = tw_timer_start (tw, interval_n);
while (42) {
tw_timer_expire (tw, expired);
foreach (e, expired) {
session = session_from_timer_id(e);
foreach (t, session->timer) {
if (t->timer_id != INVALID_ID)
tw_timer_stop (tw, t->timer_id);
t->timer_id = tw_timer_start (tw, interval);
}
}
With this sequence, tw_timer_stop might stop the wrong timer.
The problem is that tw_timer_expire removes the timer from its internal pool.
The timer_id itself is now invalid, but the timer_id stored in the external
session timer data structure is not cleared.
The following stop will find the timer_id and try to stop it. The following
start will lead to the use of a new pool entry from the wheels internal pool.
What can happen is that a pool entry release by tw_timer_expire is reused in
the loop by tw_timer_start only to be erroneously stopped by tw_timer_stop in a
later iteration of the inner loop.
Is the above explanation understandable and/or disagrees anyone with the
conclusion?
After finding the problem in my code, I wanted to see what VPP is doing. And it
seems that at least the SCTP timer code handling has this exact issue.
Regards
Andreas
--
Andreas Schultz
--
Principal Engineer
t: +49 391 819099-224
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