Re: [PATCH] md: don't create mddev in md_open

["heming.zhao@suse.com" <heming.zhao@suse.com>]

On 3/30/21 3:43 PM, Zhao Heming wrote:
> commit d3374825ce57 ("md: make devices disappear when they are no longer
> needed.") introduced protection between mddev creating & removing. The
> md_open shouldn't create mddev when all_mddevs list doesn't contain
> mddev. With currently code logic, there will be very easy to trigger
> soft lockup in non-preempt env.
> 
> *** env ***
> kvm-qemu VM 2C1G with 2 iscsi luns
> kernel should be non-preempt
> 
> *** script ***
> 
> about trigger 1 time with 10 tests
> 
> ```
> 1  node1="15sp3-mdcluster1"
> 2  node2="15sp3-mdcluster2"
> 3
> 4  mdadm -Ss
> 5  ssh ${node2} "mdadm -Ss"
> 6  wipefs -a /dev/sda /dev/sdb
> 7  mdadm -CR /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sda \
>     /dev/sdb --assume-clean
> 8
> 9  for i in {1..100}; do
> 10    echo ==== $i ====;
> 11
> 12    echo "test  ...."
> 13    ssh ${node2} "mdadm -A /dev/md0 /dev/sda /dev/sdb"
> 14    sleep 1
> 15
> 16    echo "clean  ....."
> 17    ssh ${node2} "mdadm -Ss"
> 18 done
> ```
> 

the non-clustered test script. (again: run on non-preempt kernel)

```
#!/bin/bash

mdadm -Ss
wipefs -a /dev/sda /dev/sdb
mdadm -CR /dev/md0 -b internal -e 1.2 -n 2 -l mirror /dev/sda /dev/sdb  --assume-clean
sleep 5
mdadm -Ss

for i in {1..200}; do
     echo ==== $i ====;

     echo "test  ...."
     mdadm -A /dev/md0 /dev/sda /dev/sdb
     sleep 1

     echo "clean  ....."
     mdadm -Ss
done
```

the value N on "/sys/module/md_mod/parameters/create_on_open" is useless.

Thanks,
heming