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When rcu_cbs_completed() is invoked on a non-root rcu_node structure, it unconditionally assumes that two grace periods must complete before the callbacks at hand can be invoked. This is overly conservative because if that non-root rcu_node structure believes that no grace period is in progress, and if the corresponding rcu_state structure's ->gpnum field has not yet been incremented, then these callbacks may safely be invoked after only one grace period has completed. This change is required to permit grace-period start requests to use funnel locking, which is in turn permitted to reduce root rcu_node ->lock contention, which has been observed by Nick Piggin. Furthermore, such contention will likely be increased by the merging of RCU-bh, RCU-preempt, and RCU-sched, so it makes sense to take steps to decrease it. This commit therefore improves the accuracy of rcu_cbs_completed() when invoked on a non-root rcu_node structure as described above. Reported-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Nicholas Piggin <npiggin@gmail.com>
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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.