[Tarantool-patches] [PATCH v2 4/4] qsync: don't send negative timeouts into fiber_cond_wait_timeout
Cyrill Gorcunov
gorcunov at gmail.com
Fri Jul 17 10:25:50 MSK 2020
On Fri, Jul 17, 2020 at 12:45:16AM +0200, Vladislav Shpilevoy wrote:
> Thanks for the patch!
>
> On 14.07.2020 16:53, Cyrill Gorcunov wrote:
> > Basically our timeout is calculated via (a - b), where
> > @a is a constant positive value fetched once, in turn
> > the @b is rather a dynamic value thus the result may
> > be negative. libev uses assert() call to catch such
> > values when passed to timers setup. Thus lets intercept
> > potential assert() trigger and exit early if timeout
> > is already expired.
> >
> > Signed-off-by: Cyrill Gorcunov <gorcunov at gmail.com>
> > ---
> > There were a typo, so I force-updated the branch
> >
> > src/box/txn_limbo.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
> > index d5b887d36..0924952b7 100644
> > --- a/src/box/txn_limbo.c
> > +++ b/src/box/txn_limbo.c
> > @@ -174,8 +174,10 @@ txn_limbo_wait_complete(struct txn_limbo *limbo, struct txn_limbo_entry *entry)
> > double start_time = fiber_clock();
> > while (true) {
> > double deadline = start_time + replication_synchro_timeout;
> > - bool cancellable = fiber_set_cancellable(false);
> > double timeout = deadline - fiber_clock();
> > + if (timeout < 0)
> > + goto do_rollback;
> > + bool cancellable = fiber_set_cancellable(false);
>
> I added timeout = -1; here and tried to commit a sync transaction. I
> got timed out error, no assertions. Please, tell me, how to reproduce
> the assertion you mention in the commit message. Otherwise I don't see
> why would we need the < 0 check. If it is done somewhere inside
> fiber_cond_wait_timeout anyway.
Look, here is libev code
---
noinline
void
ev_timer_start (EV_P_ ev_timer *w) EV_THROW
{
if (expect_false (ev_is_active (w)))
return;
ev_at (w) += mn_now;
assert (("libev: ev_timer_start called with negative timer repeat value", w->repeat >= 0.));
---
To trigger this assert we have to enter idle cycle, then manually change
replication_synchro_timeout via cfg (make it less than it was initially)
which should lead to negative timeout.
To be fair I don't know how to force it without error injection. Lets
assume we have initial fiber clock 1 (the clocks are increasing everytime
libev does a new polling cycle). Thus
// replication_synchro_timeout = 2
double start_time = fiber_clock(); // 1
while (true) {
double deadline = start_time + replication_synchro_timeout;
// => 3
double timeout = deadline - fiber_clock();
// => 2
bool cancellable = fiber_set_cancellable(false);
int rc = fiber_cond_wait_timeout(&limbo->wait_cond, timeout);
//
// enter into resched cycle
// manually change replication_synchro_timeout to 1
// next cycle starts
double deadline = start_time + replication_synchro_timeout;
// start_time didn't changed
// => 1 + 1 = 2
double timeout = deadline - fiber_clock();
// assume several resched cycles passed, thus
// fiber_clock returns 3, thus
// => 2 - 3 => -1
...
Am I missing something obvious here?
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