Re: [dpdk-dev] [PATCH v2] eal: fix use wrong time API

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas Monjalon
> Sent: Wednesday, May 5, 2021 8:14 AM
> 
> 04/05/2021 21:12, Morten Brørup:
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas
> Monjalon
> > > Sent: Tuesday, May 4, 2021 6:50 PM
> > >
> > > 29/04/2021 04:10, Min Hu (Connor):
> > > > Currently, the mp uses gettimeofday() API to get the time, and
> used
> > > as
> > > > timeout parameter.
> > > >
> > > > But the time which gets from gettimeofday() API isn't
> monotonically
> > > > increasing. The process may fail if the system time is changed.
> > > >
> > > > This fixes it by using clock_gettime() API with monotonic
> > > attribution.
> > > >
> > > > Fixes: 783b6e54971d ("eal: add synchronous multi-process
> > > communication")
> > > > Fixes: f05e26051c15 ("eal: add IPC asynchronous request")
> > > > Cc: stable@dpdk.org
> > > >
> > > > Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> > > > Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
> > > > ---
> > > [...]
> > > > --- a/lib/eal/common/eal_common_proc.c
> > > > +++ b/lib/eal/common/eal_common_proc.c
> > > > -	if (gettimeofday(&now, NULL) < 0) {
> > > > -		RTE_LOG(ERR, EAL, "Cannot get current time\n");
> > > > -		goto no_trigger;
> > > > -	}
> > > > -	ts_now.tv_nsec = now.tv_usec * 1000;
> > > > -	ts_now.tv_sec = now.tv_sec;
> > > > +	clock_gettime(CLOCK_MONOTONIC, &ts_now);
> > >
> > > Why not testing the return value?
> >
> > Because it is guaranteed not to fail. Ref:
> > https://linux.die.net/man/3/clock_gettime
> > https://www.freebsd.org/cgi/man.cgi?query=clock_gettime
> 
> I see "return 0 for success, or -1 for failure".
> Where is it said it cannot fail?

I'm sorry about being unclear. Referring to the "Errors" chapter in the function's man page, this function call is guaranteed not to fail with these parameters. So there is no need to check the return value.

> 
> > > I think this change would not be appropriate after -rc1.
> > > If you agree, I will postpone to DPDK 21.08.
> >
> > It does fix a serious bug, where IPC timeouts can incorrectly happen.
> And this is not a theoretical bug; I have seen errors happen due to
> using the wrong clock source in other projects.
> >
> > However, I have no clue if these IPC library functions are important
> or not. So I have no qualified opinion about postponing the change.
> 
> I think nobody hit such bug with DPDK IPC.
> 

Then I don't object to postponing.