dmu_objset_from_ds must be called with dp_config_rwlock held

[ahrens]

Motivation and Context

The normal lock order is that the dp_config_rwlock must be held before
the ds_opening_lock. For example, dmu_objset_hold() does this.
However, dmu_objset_open_impl() is called with the ds_opening_lock held,
and if the dp_config_rwlock is not already held, it will attempt to
acquire it. This may lead to deadlock, since the lock order is
reversed.

Looking at all the callers of dmu_objset_open_impl() (which is
principally the callers of dmu_objset_from_ds()), almost all callers
already have the dp_config_rwlock. However, there are a few places in
the send and receive code paths that do not. For example:
dsl_crypto_populate_key_nvlist, send_cb, dmu_recv_stream,
receive_write_byref, redact_traverse_thread.

Closes #9662

Description

This commit resolves the problem by requiring all callers to
dmu_objset_from_ds() to hold the dp_config_rwlock. In most cases, the
code has been restructured such that we call dmu_objset_from_ds()
earlier on in the send and receive processes, when we already have the
dp_config_rwlock, and save the objset_t until we need it in the middle
of the send or receive (similar to what we already do with the
dsl_dataset_t). Thus we do not need to acquire the dp_config_rwlock in
many new places.

I also cleaned up code in dmu_redact_snap() and send_traverse_thread().

How Has This Been Tested?

test suite

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.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[PaulZ-98]

Looks good. Comments make it pretty clear. I'm not too familiar with the redacted send code so that part could probably use another set of eyes.

[behlendorf]

@freqlabs I wanted to make sure you saw this. After this is merged you'll want to make similar minor updates to the FreeBSD zfs_resume_fs and zfs_end_fs functions.

[ghost]

@freqlabs I wanted to make sure you saw this. After this is merged you'll want to make similar minor updates to the FreeBSD zfs_resume_fs and zfs_end_fs functions.

Thanks, I appreciate the heads-up.