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The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks counter of an incoming CPU when required. It is currently invoked from rcutree_prepare_cpu(), which runs before the incoming CPU is running, and thus on some other CPU. This makes the per-CPU accesses in rcu_dynticks_eqs_online() iffy at best, and it all "works" only because the running CPU cannot possibly be in dyntick-idle mode, which means that rcu_dynticks_eqs_online() never has any effect. It is currently OK for rcu_dynticks_eqs_online() to have no effect, but only because the CPU-offline process just happens to leave ->dynticks in the correct state. After all, if ->dynticks were in the wrong state on a just-onlined CPU, rcutorture would complain bitterly the next time that CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y, for example, those built by rcutorture scenario TREE04. One could argue that this means that rcu_dynticks_eqs_online() is unnecessary, however, removing it would make the CPU-online process vulnerable to slight changes in the CPU-offline process. One could also ask why it is safe to move the rcu_dynticks_eqs_online() call so late in the CPU-online process. Indeed, there was a time when it would not have been safe, which does much to explain its current location. However, the marking of a CPU as online from an RCU perspective has long since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all that is required is that ->dynticks be set correctly by the time that the CPU is marked as online from an RCU perspective. After all, the RCU grace-period kthread does not check to see if offline CPUs are also idle. (In case you were curious, this is one reason why there is quiescent-state reporting as part of the offlining process.) This commit therefore moves the call to rcu_dynticks_eqs_online() from rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed to be running on the incoming CPU. The call to this function must of course be placed before this rcu_cpu_starting() announces this CPU's presence to RCU. Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Paul E. McKenney <paulmck@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.