thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed'

[abdelmonem17]

hello everyone I am using project called toshi from crate.io and it uses tokio which works well under windows 10 but when I compiled and run it under VM 'Ubuntu 20.04.2 LTS' always after few seconds the program stop responses and gives me a panic like the following

thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /home/my-user/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.2.0/src/time/driver/mod.rs:260:9

this error also exist in the old version that uses old version of tokio and the output of the panic is:

thread 'tokio-runtime-worker' panicked at 'elapsed=71666; when=71665', /home/my-user/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.25/src/time/wheel/mod.rs:223:9
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

[carllerche]

Hmm, it looks like this is due to Instant::now() returning past values, which should not be possible. Looking at the code, I don't see any other possible reason short of memory corruption which seems unlikely. Can you see if you can get your VM to produce an Instant::now() that decrements? It may require involving multiple threads.

[taiki-e]

I guess this is the same issue as rust-lang/rust#60475.

• On Windows, timer is not monotonic, so std forces Instant::now to be monotonic.
• On Linux, timer is monotonic, so std does not force Instant::now to be monotonic.
• VM is probably implemented to use the host OS's timer. So Linux's timer on the VM is the same as the Windows's timer, and it is not monotonic. (Or it's not monotonic due to a bug.) But since the OS is Linux, std does not force Instant::now to be monotonic. As a result, Instant::now may returns past values.

[carllerche]

Yeah, seems likely. I guess we should just accept Instant::now() can be buggy and defend against it.

[abdelmonem17]

@carllerche , @taiki-e is there a workaround to fix this issue ?

[Darksonn]

You could try to remove the assert that panicked, but it might fail in other less obvious ways. The code wasn't written to behave properly when time goes backwards.

[abdelmonem17]

@Darksonn I download the crate and remove the line but I have a panic because it fails that the forked library is different runtime than tokio.

[Darksonn]

You need to use [patch.crates-io] in order for the library to recognize your fork as the right version of Tokio.

[abdelmonem17]

@Darksonn thank you for your help.

[taiki-e]

related rust-lang/rust#84448

[jgrund]

FWIW, I'm seeing this happen across multiple tokio instances quite frequently.

My host is CentOS 7.9.2009 and my guests are CentOS 7.9.2009 running in VirtualBox.

My current_clocksource on both the host and the guests are tsc.

[root@node1 ~]# clush -ab cat /var/log/messages | grep panic
May 22 11:31:12 node3 emf-device-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 10:01:57 node4 emf-snapshot-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 00:55:28 node5 emf-ostpool-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 07:31:33 node5 emf-ntp-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 05:31:12 node6 emf-host-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 05:52:47 node6 emf-network-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 06:19:52 node6 emf-device-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9
May 22 05:15:36 node8 emf-corosync-agent: thread 'tokio-runtime-worker' panicked at 'assertion failed: now >= lock.elapsed', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.6.0/src/time/driver/mod.rs:267:9

[carllerche]

@jgrund do you want to submit a PR that avoids the panic and instead ignores Instant::now() values that are in the past?

Feel free to ping someone in #tokio-dev on Discord (https://discord.gg/tokio) if you need guidance.

[jgrund]

@carllerche It turned out this was actually useful in my case. It led me to update some kernel parameters in my vm which stopped the clock from warping (which was effecting other processes as well).

[carllerche]

@jgrund Do you want to submit a PR that improves the assertion message? Maybe explain the problem + include links (e.g. rust-lang/rust#84448) and help on how to fix it?

[NeoLegends]

Is this something where the std should be patched to also defend against non-monotonic clocks on Linux?

[Darksonn]

It is a bug in std that this can happen, yes.

[the8472]

It is a bug in std that this can happen, yes.

Well technically it is a kernel or hypervisor bug and should be reported upstream to whatever vendor is causing this. std relies on CLOCK_MONOTONIC guarantees from the OS and the OS may either perform the monotonization itself or rely on the hypervisor to signal clock capabilities correctly.

@carllerche It turned out this was actually useful in my case. It led me to update some kernel parameters in my vm which stopped the clock from warping (which was effecting other processes as well).

Yeah, picking an unreliable clock source manually could also cause this. By default the kernel should pick a reliable one. If it isn't that should be reported.