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Due to the asynchronous nature of the dlm api, when we request a pending locking request to be canceled with dlm_unlock(DLM_LKF_CANCEL), the locking request will either complete before it could be canceled, or the cancellation will succeed. In either case, gdlm_ast will be called once and the status will indicate the outcome of the locking request, with -DLM_ECANCEL indicating a canceled request. Inside dlm, when a locking request completes before its cancel request could be processed, gdlm_ast will be called, but the lock will still be considered busy until a DLM_MSG_CANCEL_REPLY message completes the cancel request. During that time, successive dlm_lock() or dlm_unlock() requests for that lock will return -EBUSY. In other words, waiting for the gdlm_ast call before issuing the next locking request is not enough. There is no way of waiting for a cancel request to actually complete, either. We rarely cancel locking requests, but when we do, we don't know when the next locking request for that lock will occur. This means that any dlm_lock() or dlm_unlock() call can potentially return -EBUSY. When that happens, this patch simply repeats the request after a short pause. This workaround could be improved upon by tracking for which dlm locks cancel requests have been issued, but that isn't strictly necessary and it would complicate the code. We haven't seen -EBUSY errors from dlm without cancel requests. Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com> |
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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.