Reschedule processes on error.

[ab-oe]

Description

On the single core machine the system may hang when the spa_namespare_lock acquisition fails in the zvol_first_open function. It returns ERESTARTSYS error what causes the endless loop in __blkdev_get function.

Motivation and Context

Solves system hang on single core machines.

How Has This Been Tested?

On KVM single core virtual machine following test has been run:

zpool create p0 sdb
for i in `seq 1 100`; do zfs create -s -V10G p0/zvi${1}; done
zpool export p0
zpool import p0

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[mention-bot]

@ab-oe, thanks for your PR! By analyzing the history of the files in this pull request, we identified @behlendorf, @bprotopopov and @tuxoko to be potential reviewers.

[kernelOfTruth]

@behlendorf I've added this to 0.7.0-rc5 since this is a potential stability/reliability blocker for UP boxes (hang), hope that's okay

edit:
the issue (#6283) is there already anyway - so it's more or less redundant

[behlendorf]

Let's restrict this to the -ERESTARTSYS case. This makes it clear exactly why this additional schedule call is here, all other error values should be returned to sys_open().

if (error == -ERESTARTSYS)
       schedule();