MFC r286689: 5981 Deadlock in dmu_objset_find_dp

illumos/illumos-gate@1d3f896f5469c69c1339890ec3d68e9feddb0343

https://www.illumos.org/issues/5981
When dmu_objset_find_dp gets called with a read lock held, it fans out
the work to the task queue. Each task in turn acquires its own read
lock before calling the callback. If during this process anyone tries
to a acquire a write lock, it will stall all read lock requests.Thus
the tasks will never finish, the read lock of the caller will never
get freed and the write lock never acquired. deadlock.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Arne Jansen <jansen@webgods.de>

MFC r268865: MFV r268852:

Reduce lock contention on the z_teardown_lock under heavily cached
read workload by splitting the single teardown rrw lock into
RRM_NUM_LOCKS (17) of them.

Read acquisitions are randomly distributed among these locks based
on curthread pointer. Write acquisitions are going to all the
locks, which for the usage of this type of lock should be rare.

Illumos issue:
5008 lock contention (rrw_exit) while running a read only load

MFC r286689: 5981 Deadlock in dmu_objset_find_dp

illumos/illumos-gate@1d3f896f5469c69c1339890ec3d68e9feddb0343

https://www.illumos.org/issues/5981
When dmu_objset_find_dp gets called with a read lock held, it fans out
the work to the task queue. Each task in turn acquires its own read
lock before calling the callback. If during this process anyone tries
to a acquire a write lock, it will stall all read lock requests.Thus
the tasks will never finish, the read lock of the caller will never
get freed and the write lock never acquired. deadlock.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Arne Jansen <jansen@webgods.de>

MFC r268865: MFV r268852:

Reduce lock contention on the z_teardown_lock under heavily cached
read workload by splitting the single teardown rrw lock into
RRM_NUM_LOCKS (17) of them.

Read acquisitions are randomly distributed among these locks based
on curthread pointer. Write acquisitions are going to all the
locks, which for the usage of this type of lock should be rare.

Illumos issue:
5008 lock contention (rrw_exit) while running a read only load