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rcu: Remove RCU_NONIDLE()

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Since there are now exactly _zero_ users of RCU_NONIDLE(), make it go
away before someone else decides to (ab)use it.

[ paulmck: Remove extraneous whitespace. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Peter Zijlstra 2023-03-20 18:37:51 +01:00 committed by Paul E. McKenney
parent 1da82598cf
commit 7a3cc29136
3 changed files with 1 additions and 61 deletions
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36
Documentation/RCU/Design/Requirements/Requirements.rst
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@ -2071,41 +2071,7 @@ call.
Because RCU avoids interrupting idle CPUs, it is illegal to execute an
RCU read-side critical section on an idle CPU. (Kernels built with
``CONFIG_PROVE_RCU=y`` will splat if you try it.) The RCU_NONIDLE()
macro and ``_rcuidle`` event tracing is provided to work around this
restriction. In addition, rcu_is_watching() may be used to test
whether or not it is currently legal to run RCU read-side critical
sections on this CPU. I learned of the need for diagnostics on the one
hand and RCU_NONIDLE() on the other while inspecting idle-loop code.
Steven Rostedt supplied ``_rcuidle`` event tracing, which is used quite
heavily in the idle loop. However, there are some restrictions on the
code placed within RCU_NONIDLE():
#. Blocking is prohibited. In practice, this is not a serious
restriction given that idle tasks are prohibited from blocking to
begin with.
#. Although nesting RCU_NONIDLE() is permitted, they cannot nest
indefinitely deeply. However, given that they can be nested on the
order of a million deep, even on 32-bit systems, this should not be a
serious restriction. This nesting limit would probably be reached
long after the compiler OOMed or the stack overflowed.
#. Any code path that enters RCU_NONIDLE() must sequence out of that
same RCU_NONIDLE(). For example, the following is grossly
illegal:
::
1 RCU_NONIDLE({
2 do_something();
3 goto bad_idea; /* BUG!!! */
4 do_something_else();});
5 bad_idea:
It is just as illegal to transfer control into the middle of
RCU_NONIDLE()'s argument. Yes, in theory, you could transfer in
as long as you also transferred out, but in practice you could also
expect to get sharply worded review comments.
``CONFIG_PROVE_RCU=y`` will splat if you try it.)
It is similarly socially unacceptable to interrupt an ``nohz_full`` CPU
running in userspace. RCU must therefore track ``nohz_full`` userspace

1
Documentation/RCU/whatisRCU.rst
View File

@ -1117,7 +1117,6 @@ All: lockdep-checked RCU utility APIs::
RCU_LOCKDEP_WARN
rcu_sleep_check
RCU_NONIDLE
All: Unchecked RCU-protected pointer access::

25
include/linux/rcupdate.h
View File

@ -156,31 +156,6 @@ static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
static inline void rcu_nocb_flush_deferred_wakeup(void) { }
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
/**
* RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
* @a: Code that RCU needs to pay attention to.
*
* RCU read-side critical sections are forbidden in the inner idle loop,
* that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU
* will happily ignore any such read-side critical sections. However,
* things like powertop need tracepoints in the inner idle loop.
*
* This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
* will tell RCU that it needs to pay attention, invoke its argument
* (in this example, calling the do_something_with_RCU() function),
* and then tell RCU to go back to ignoring this CPU. It is permissible
* to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
* on the order of a million or so, even on 32-bit systems). It is
* not legal to block within RCU_NONIDLE(), nor is it permissible to
* transfer control either into or out of RCU_NONIDLE()'s statement.
*/
#define RCU_NONIDLE(a) \
do { \
ct_irq_enter_irqson(); \
do { a; } while (0); \
ct_irq_exit_irqson(); \
} while (0)
/*
* Note a quasi-voluntary context switch for RCU-tasks's benefit.
* This is a macro rather than an inline function to avoid #include hell.