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commit0b207d02bdupstream. rbd_dev_refresh() has been holding header_rwsem across header and parent info read-in unnecessarily for ages. With commit870611e487("rbd: get snapshot context after exclusive lock is ensured to be held"), the potential for deadlocks became much more real owning to a) header_rwsem now nesting inside lock_rwsem and b) rw_semaphores not allowing new readers after a writer is registered. For example, assuming that I/O request 1, I/O request 2 and header read-in request all target the same OSD: 1. I/O request 1 comes in and gets submitted 2. watch error occurs 3. rbd_watch_errcb() takes lock_rwsem for write, clears owner_cid and releases lock_rwsem 4. after reestablishing the watch, rbd_reregister_watch() calls rbd_dev_refresh() which takes header_rwsem for write and submits a header read-in request 5. I/O request 2 comes in: after taking lock_rwsem for read in __rbd_img_handle_request(), it blocks trying to take header_rwsem for read in rbd_img_object_requests() 6. another watch error occurs 7. rbd_watch_errcb() blocks trying to take lock_rwsem for write 8. I/O request 1 completion is received by the messenger but can't be processed because lock_rwsem won't be granted anymore 9. header read-in request completion can't be received, let alone processed, because the messenger is stranded Change rbd_dev_refresh() to take header_rwsem only for actually updating rbd_dev->header. Header and parent info read-in don't need any locking. Cc: stable@vger.kernel.org #0b035401c5: rbd: move rbd_dev_refresh() definition Cc: stable@vger.kernel.org #510a7330c8: rbd: decouple header read-in from updating rbd_dev->header Cc: stable@vger.kernel.org #c10311776f: rbd: decouple parent info read-in from updating rbd_dev Cc: stable@vger.kernel.org Fixes:870611e487("rbd: get snapshot context after exclusive lock is ensured to be held") Signed-off-by: Ilya Dryomov <idryomov@gmail.com> Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.