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This is done by creating a new RCU data structure (io_ev_fd) as part of io_ring_ctx that holds the eventfd_ctx. The function io_eventfd_signal is executed under rcu_read_lock with a single rcu_dereference to io_ev_fd so that if another thread unregisters the eventfd while io_eventfd_signal is still being executed, the eventfd_signal for which io_eventfd_signal was called completes successfully. The process of registering/unregistering eventfd is already done under uring_lock so multiple threads won't enter a race condition while registering/unregistering eventfd. With the above approach ring quiesce can be avoided which is much more expensive then using RCU lock. On the system tested, io_uring_register with IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms before with ring quiesce. Signed-off-by: Usama Arif <usama.arif@bytedance.com> Link: https://lore.kernel.org/r/20220204145117.1186568-3-usama.arif@bytedance.com [axboe: long line fixups] Signed-off-by: Jens Axboe <axboe@kernel.dk>
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.