[Samba] Samba 4.7.6-ubuntu taking 100% CPU

[Jeremy Allison]

On Fri, Apr 02, 2021 at 06:56:47PM +0200, Olaf Marzocchi via samba wrote:
>Dear all,
>I already sent this email but I wrongly added attachments, resulting 
>in the first email being rejected. Sorry if you read the followup with 
>ZFS properties.
>I'm sending it again using external storage where needed.
>
>I recently incurred in an issue with Samba 4.7.6-ubuntu (Ubuntu 
>18.04.05 LTS, with latest updates).
>I'm not sure what caused the issue, since at the same time I performed 
>updates to Ubuntu (apt upgrade) and I separately updated the zfs 
>kernel module to 2.0.4 from whatever version available mid last year, 
>also some time passed before I noticed the present issue.
>
>The issue manifests itself as following: when I try to open some files 
>(not all of them) from any SMB share (I tried two different Windows 10 
>machines and one macOS 10.14), the smb3 process takes 100% CPU, it 
>doesn't seem to retrieve the file, and I have to kill it "-9", the 
>hard way. I found that, sometimes, after I kill smbd3 Windows gets the 
>file I tried to open, but it's not a way that always works: sometimes 
>it's corrupted (cut too short).
>I tested locally from ssh and the same files have no issues 
>whatsoever, it's not a hardware issue. I can read and copy them 
>without problems.

Use strace -p <spinning process id> to get system call
traces to see what is going on.