The following commit has been merged into the core/rcu branch of tip: Commit-ID: 81ad58be2f83f9bd675f67ca5b8f420358ddf13c Gitweb: https://git.kernel.org/tip/81ad58be2f83f9bd675f67ca5b8f420358ddf13c Author: Sebastian Andrzej Siewior <bige...@linutronix.de> AuthorDate: Tue, 15 Dec 2020 15:16:49 +01:00 Committer: Paul E. McKenney <paul...@kernel.org> CommitterDate: Wed, 06 Jan 2021 16:10:44 -08:00
doc: Use CONFIG_PREEMPTION CONFIG_PREEMPTION is selected by CONFIG_PREEMPT and by CONFIG_PREEMPT_RT. Both PREEMPT and PREEMPT_RT require the same functionality which today depends on CONFIG_PREEMPT. Update the documents and mention CONFIG_PREEMPTION. Spell out CONFIG_PREEMPT_RT (instead PREEMPT_RT) since it is an option now. Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de> Signed-off-by: Paul E. McKenney <paul...@kernel.org> --- Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst | 4 ++-- Documentation/RCU/Design/Requirements/Requirements.rst | 22 +++++++++++----------- Documentation/RCU/checklist.rst | 2 +- Documentation/RCU/rcubarrier.rst | 6 +++--- Documentation/RCU/stallwarn.rst | 4 ++-- Documentation/RCU/whatisRCU.rst | 10 +++++----- 6 files changed, 24 insertions(+), 24 deletions(-) diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst index 72f0f6f..6f89cf1 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst @@ -38,7 +38,7 @@ sections. RCU-preempt Expedited Grace Periods =================================== -``CONFIG_PREEMPT=y`` kernels implement RCU-preempt. +``CONFIG_PREEMPTION=y`` kernels implement RCU-preempt. The overall flow of the handling of a given CPU by an RCU-preempt expedited grace period is shown in the following diagram: @@ -112,7 +112,7 @@ things. RCU-sched Expedited Grace Periods --------------------------------- -``CONFIG_PREEMPT=n`` kernels implement RCU-sched. The overall flow of +``CONFIG_PREEMPTION=n`` kernels implement RCU-sched. The overall flow of the handling of a given CPU by an RCU-sched expedited grace period is shown in the following diagram: diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index bac1cdd..42a81e3 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -78,7 +78,7 @@ RCU treats a nested set as one big RCU read-side critical section. Production-quality implementations of rcu_read_lock() and rcu_read_unlock() are extremely lightweight, and in fact have exactly zero overhead in Linux kernels built for production use with -``CONFIG_PREEMPT=n``. +``CONFIG_PREEMPTION=n``. This guarantee allows ordering to be enforced with extremely low overhead to readers, for example: @@ -1181,7 +1181,7 @@ and has become decreasingly so as memory sizes have expanded and memory costs have plummeted. However, as I learned from Matt Mackall's `bloatwatch <http://elinux.org/Linux_Tiny-FAQ>`__ efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (``CONFIG_PREEMPT=n``) kernels, and thus `tiny +non-preemptible (``CONFIG_PREEMPTION=n``) kernels, and thus `tiny RCU <https://lore.kernel.org/r/20090113221724.ga15...@linux.vnet.ibm.com>`__ was born. Josh Triplett has since taken over the small-memory banner with his `Linux kernel tinification <https://tiny.wiki.kernel.org/>`__ @@ -1497,7 +1497,7 @@ limitations. Implementations of RCU for which rcu_read_lock() and rcu_read_unlock() generate no code, such as Linux-kernel RCU when -``CONFIG_PREEMPT=n``, can be nested arbitrarily deeply. After all, there +``CONFIG_PREEMPTION=n``, can be nested arbitrarily deeply. After all, there is no overhead. Except that if all these instances of rcu_read_lock() and rcu_read_unlock() are visible to the compiler, compilation will eventually fail due to exhausting memory, @@ -1769,7 +1769,7 @@ implementation can be a no-op. However, once the scheduler has spawned its first kthread, this early boot trick fails for synchronize_rcu() (as well as for -synchronize_rcu_expedited()) in ``CONFIG_PREEMPT=y`` kernels. The +synchronize_rcu_expedited()) in ``CONFIG_PREEMPTION=y`` kernels. The reason is that an RCU read-side critical section might be preempted, which means that a subsequent synchronize_rcu() really does have to wait for something, as opposed to simply returning immediately. @@ -2038,7 +2038,7 @@ the following: 5 rcu_read_unlock(); 6 do_something_with(v, user_v); -If the compiler did make this transformation in a ``CONFIG_PREEMPT=n`` kernel +If the compiler did make this transformation in a ``CONFIG_PREEMPTION=n`` kernel build, and if get_user() did page fault, the result would be a quiescent state in the middle of an RCU read-side critical section. This misplaced quiescent state could result in line 4 being a use-after-free access, @@ -2320,7 +2320,7 @@ conjunction with the `-rt patchset <https://wiki.linuxfoundation.org/realtime/>`__. The real-time-latency response requirements are such that the traditional approach of disabling preemption across RCU read-side critical sections -is inappropriate. Kernels built with ``CONFIG_PREEMPT=y`` therefore use +is inappropriate. Kernels built with ``CONFIG_PREEMPTION=y`` therefore use an RCU implementation that allows RCU read-side critical sections to be preempted. This requirement made its presence known after users made it clear that an earlier `real-time @@ -2460,11 +2460,11 @@ not have this property, given that any point in the code outside of an RCU read-side critical section can be a quiescent state. Therefore, *RCU-sched* was created, which follows “classic” RCU in that an RCU-sched grace period waits for pre-existing interrupt and NMI -handlers. In kernels built with ``CONFIG_PREEMPT=n``, the RCU and +handlers. In kernels built with ``CONFIG_PREEMPTION=n``, the RCU and RCU-sched APIs have identical implementations, while kernels built with -``CONFIG_PREEMPT=y`` provide a separate implementation for each. +``CONFIG_PREEMPTION=y`` provide a separate implementation for each. -Note well that in ``CONFIG_PREEMPT=y`` kernels, +Note well that in ``CONFIG_PREEMPTION=y`` kernels, rcu_read_lock_sched() and rcu_read_unlock_sched() disable and re-enable preemption, respectively. This means that if there was a preemption attempt during the RCU-sched read-side critical section, @@ -2627,10 +2627,10 @@ userspace execution also delimit tasks-RCU read-side critical sections. The tasks-RCU API is quite compact, consisting only of call_rcu_tasks(), synchronize_rcu_tasks(), and -rcu_barrier_tasks(). In ``CONFIG_PREEMPT=n`` kernels, trampolines +rcu_barrier_tasks(). In ``CONFIG_PREEMPTION=n`` kernels, trampolines cannot be preempted, so these APIs map to call_rcu(), synchronize_rcu(), and rcu_barrier(), respectively. In -``CONFIG_PREEMPT=y`` kernels, trampolines can be preempted, and these +``CONFIG_PREEMPTION=y`` kernels, trampolines can be preempted, and these three APIs are therefore implemented by separate functions that check for voluntary context switches. diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst index 2d1dc1d..1030119 100644 --- a/Documentation/RCU/checklist.rst +++ b/Documentation/RCU/checklist.rst @@ -212,7 +212,7 @@ over a rather long period of time, but improvements are always welcome! the rest of the system. 7. As of v4.20, a given kernel implements only one RCU flavor, - which is RCU-sched for PREEMPT=n and RCU-preempt for PREEMPT=y. + which is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. If the updater uses call_rcu() or synchronize_rcu(), then the corresponding readers may use rcu_read_lock() and rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(), diff --git a/Documentation/RCU/rcubarrier.rst b/Documentation/RCU/rcubarrier.rst index f64f441..3b4a248 100644 --- a/Documentation/RCU/rcubarrier.rst +++ b/Documentation/RCU/rcubarrier.rst @@ -9,7 +9,7 @@ RCU (read-copy update) is a synchronization mechanism that can be thought of as a replacement for read-writer locking (among other things), but with very low-overhead readers that are immune to deadlock, priority inversion, and unbounded latency. RCU read-side critical sections are delimited -by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT +by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPTION kernels, generate no code whatsoever. This means that RCU writers are unaware of the presence of concurrent @@ -329,10 +329,10 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last to smp_call_function() and further to smp_call_function_on_cpu(), causing this latter to spin until the cross-CPU invocation of rcu_barrier_func() has completed. This by itself would prevent - a grace period from completing on non-CONFIG_PREEMPT kernels, + a grace period from completing on non-CONFIG_PREEMPTION kernels, since each CPU must undergo a context switch (or other quiescent state) before the grace period can complete. However, this is - of no use in CONFIG_PREEMPT kernels. + of no use in CONFIG_PREEMPTION kernels. Therefore, on_each_cpu() disables preemption across its call to smp_call_function() and also across the local call to diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst index c9ab6af..e97d1b4 100644 --- a/Documentation/RCU/stallwarn.rst +++ b/Documentation/RCU/stallwarn.rst @@ -25,7 +25,7 @@ warnings: - A CPU looping with bottom halves disabled. -- For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel +- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel without invoking schedule(). If the looping in the kernel is really expected and desirable behavior, you might need to add some calls to cond_resched(). @@ -44,7 +44,7 @@ warnings: result in the ``rcu_.*kthread starved for`` console-log message, which will include additional debugging information. -- A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might +- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might happen to preempt a low-priority task in the middle of an RCU read-side critical section. This is especially damaging if that low-priority task is not permitted to run on any other CPU, diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst index 1a4723f..17e95ab 100644 --- a/Documentation/RCU/whatisRCU.rst +++ b/Documentation/RCU/whatisRCU.rst @@ -683,7 +683,7 @@ Quick Quiz #1: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and -on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT +on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. :: @@ -739,7 +739,7 @@ Quick Quiz #2: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? :ref:`Answers to Quick Quiz <8_whatisRCU>` @@ -1093,7 +1093,7 @@ Quick Quiz #2: overhead is **negative**. Answer: - Imagine a single-CPU system with a non-CONFIG_PREEMPT + Imagine a single-CPU system with a non-CONFIG_PREEMPTION kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling @@ -1120,10 +1120,10 @@ Answer: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? Answer: - Just as PREEMPT_RT permits preemption of spinlock + Just as CONFIG_PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical
