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e62d8ae462
The rcu_tasks_need_gpcb() samples ->percpu_dequeue_lim as part of the condition clause of a "for" loop, which is a bit confusing. This commit therefore hoists this sampling out of the loop, using the result loaded in the condition clause. So why does this work in the face of a concurrent switch from single-CPU queueing to per-CPU queueing? o The call_rcu_tasks_generic() that makes the change has already enqueued its callback, which means that all of the other CPU's callback queues are empty. o For the call_rcu_tasks_generic() that first notices the switch to per-CPU queues, the smp_store_release() used to update ->percpu_enqueue_lim pairs with the raw_spin_trylock_rcu_node()'s full barrier that is between the READ_ONCE(rtp->percpu_enqueue_shift) and the rcu_segcblist_enqueue() that enqueues the callback. o Because this CPU's queue is empty (unless it happens to be the original single queue, in which case there is no need for synchronization), this call_rcu_tasks_generic() will do an irq_work_queue() to schedule a handler for the needed rcuwait_wake_up() call. This call will be ordered after the first call_rcu_tasks_generic() function's change to ->percpu_dequeue_lim. o This rcuwait_wake_up() will either happen before or after the set_current_state() in rcuwait_wait_event(). If it happens before, the "condition" argument's call to rcu_tasks_need_gpcb() will be ordered after the original change, and all callbacks on all CPUs will be visible. Otherwise, if it happens after, then the grace-period kthread's state will be set back to running, which will result in a later call to rcuwait_wait_event() and thus to rcu_tasks_need_gpcb(), which will again see the change. So it all works out. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> |
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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.