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bf0a57445d
A CPU coming online checks for an ongoing grace period and reports
a quiescent state accordingly if needed. This special treatment that
shortcuts the expedited IPI finds its origin as an optimization purpose
on the following commit:
338b0f760e (rcu: Better hotplug handling for synchronize_sched_expedited()
The point is to avoid an IPI while waiting for a CPU to become online
or failing to become offline.
However this is pointless and even error prone for several reasons:
* If the CPU has been seen offline in the first round scanning offline
and idle CPUs, no IPI is even tried and the quiescent state is
reported on behalf of the CPU.
* This means that if the IPI fails, the CPU just became offline. So
it's unlikely to become online right away, unless the cpu hotplug
operation failed and rolled back, which is a rare event that can
wait a jiffy for a new IPI to be issued.
* But then the "optimization" applying on failing CPU hotplug down only
applies to !PREEMPT_RCU.
* This force reports a quiescent state even if ->cpu_no_qs.b.exp is not
set. As a result it can race with remote QS reports on the same rdp.
Fortunately it happens to be OK but an accident is waiting to happen.
For all those reasons, remove this optimization that doesn't look worthy
to keep around.
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@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 reStructuredText 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.