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dsl_pool: extend comment on DSL Pool Configuration Lock 

Based on a conversation with Matt on the OpenZFS Slack.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11370
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Christian Schwarz 2020-12-20 02:04:05 +00:00 committed by GitHub
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1 changed files with 29 additions and 2 deletions
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module/zfs/dsl_pool.c
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@ -1270,8 +1270,16 @@ dsl_pool_user_release(dsl_pool_t *dp, uint64_t dsobj, const char *tag,
* (e.g. it could be destroyed). Therefore you shouldn't do anything to the
* dataset except release it.
*
* User-initiated operations (e.g. ioctls, zfs_ioc_*()) are either read-only
* or modifying operations.
* Operations generally fall somewhere into the following taxonomy:
*
* Read-Only Modifying
*
* Dataset Layer / MOS zfs get zfs destroy
*
* Individual Dataset read() write()
*
*
* Dataset Layer Operations
*
* Modifying operations should generally use dsl_sync_task(). The synctask
* infrastructure enforces proper locking strategy with respect to the
@ -1281,6 +1289,25 @@ dsl_pool_user_release(dsl_pool_t *dp, uint64_t dsobj, const char *tag,
* information from the dataset, then release the pool and dataset.
* dmu_objset_{hold,rele}() are convenience routines that also do the pool
* hold/rele.
*
*
* Operations On Individual Datasets
*
* Objects _within_ an objset should only be modified by the current 'owner'
* of the objset to prevent incorrect concurrent modification. Thus, use
* {dmu_objset,dsl_dataset}_own to mark some entity as the current owner,
* and fail with EBUSY if there is already an owner. The owner can then
* implement its own locking strategy, independent of the dataset layer's
* locking infrastructure.
* (E.g., the ZPL has its own set of locks to control concurrency. A regular
* vnop will not reach into the dataset layer).
*
* Ideally, objects would also only be read by the objset’s owner, so that we
* don’t observe state mid-modification.
* (E.g. the ZPL is creating a new object and linking it into a directory; if
* you don’t coordinate with the ZPL to hold ZPL-level locks, you could see an
* intermediate state. The ioctl level violates this but in pretty benign
* ways, e.g. reading the zpl props object.)
*/
int