zramctl fails if called within a certain interval after modprobe zram.

[hackerb9]

On my machine, I can trigger the bug 100% of the time if I do this:
  modprobe zram; d=$(zramctl --find) && zramctl --size 256M $d

Here is the error message zramctl emits:
  zramctl: /dev/zram0: failed to reset: Device or resource busy

However, the following always work (0% failure)!
  modprobe zram; sleep 0.1; d=$(zramctl --find) && zramctl --size 256M $d
  modprobe zram; d=$(zramctl --find) && sleep 0.1 && zramctl --size 256M $d

And the following sometimes works and sometimes doesn't:
  modprobe zram; d=$(zramctl --find --size 256M)
Chances are about 25% of failure according to this:
    for i in {0..99};
    do
      echo $i;
      modprobe -r zram;
      modprobe zram;
      d=$(zramctl --find --size 256M);
      sleep 0.05;
    done 2>&1 | grep -c failed

Bizarrely, while putting a long sleep (0.1s) after the modprobe fixes
the problem, putting a short sleep in (0.01s) increases the failure
rate to 95%!

----------------------------------------------------------------------
    for i in {0..99};
    do
      echo $i;
      modprobe -r zram;
      modprobe zram;
      sleep 0.01 # <-- this makes things worse!
      d=$(zramctl --find --size 256M);
      sleep 0.05;
    done 2>&1 | grep -c failed
----------------------------------------------------------------------
    for i in {0..99};
    do
      echo $i;
      modprobe -r zram;
      modprobe zram;
      sleep 0.10 # <-- this completely fixes the problem
      d=$(zramctl --find --size 256M);
      sleep 0.05;
    done 2>&1 | grep -c failed
----------------------------------------------------------------------

Because of the weird window of time in which this bug occurs, I assume it is caused by some asynchronous process briefly locking the device shortly after 'modprobe zram'. I'm guessing zramctl's --find part is running before the lock but the --size part happens while the device is in use.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Björn (bjmi)]

Thank you very much for sharing your findings, now I can stop troubleshooting my script.
Your proposed workaround with sleep 0.1 works.
I'm affected by this bug in UnraidOS that is based on Slackware Linux.
I got "zramctl: /dev/zram0: failed to reset: Device or resource busy" when
    modprobe zram
    zramctl -f -s 8G -a zstd
was used.