Update:
I've pimped the test program a bit and ran it against a local CIFS share
like Konstantin did.

Output of some of my test runs:

$ ./its-raining-signals /mnt/shared 100
100 signals during 747470 loop iterations
100 signals during 752470 loop iterations
100 signals during 733729 loop iterations
100 signals during 754084 loop iterations
100 signals during 746603 loop iterations
stat() failed: Interrupted system call

$ ./its-raining-signals /mnt/shared 1000
1000 signals during 746063 loop iterations
stat() failed: Interrupted system call

$ ./its-raining-signals /mnt/shared 10000
stat() failed: Interrupted system call

$ ./its-raining-signals /mnt/shared 1000 r
1000 signals during 724637 loop iterations
1000 signals during 749337 loop iterations
1000 signals during 747301 loop iterations
[... keeps on going, no errors ...]

$ ./its-raining-signals /mnt/shared 10000 r
[no output, hangs]

These are the two CIFS-related kernel patches I've found so far:
- cifs: handle -EINTR in cifs_setattr
https://github.com/torvalds/linux/commit/c6cc4c5a72505a0ecefc9b413f16bec512f38078
   (earliest linux-stable tag including the patch: v5.10-rc1)
- cifs: do not fail __smb_send_rqst if non-fatal signals are pending
   (already mentioned by Godmar)
https://github.com/torvalds/linux/commit/214a5ea081e77346e4963dd6d20c5539ff8b6ae6
   (earliest linux-stable tag including the patch: v5.11-rc5)

If I checked the Ubuntu focal git repo correctly, both commits aren't in
there, so apparently no backports yet in Ubuntu.

I'll check how much effort it is to install a newer mainline kernel
(including the kernel modules like cifs.ko) in Ubuntu focal.

Tobias