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Commit cae45e1c authored by Ingo Molnar's avatar Ingo Molnar
Browse files

Merge branch 'rcu-next' of... 

Merge branch 'rcu-next' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

 into core/rcu

Pull the latest RCU tree from Paul E. McKenney:

 - Additional cleanups after RCU flavor consolidation
 - Grace-period forward-progress cleanups and improvements
 - Documentation updates
 - Miscellaneous fixes
 - spin_is_locked() conversions to lockdep
 - SPDX changes to RCU source and header files
 - SRCU updates
 - Torture-test updates, including nolibc updates and moving
   nolibc to tools/include

Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parents aa0c38cf e7ffb4eb

Documentation/RCU/Design/Expedited-Grace-Periods/ExpSchedFlow.svg

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Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html

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@@ -72,10 +72,10 @@ will ignore it because idle and offline CPUs are already residing 
in quiescent states.

Otherwise, the expedited grace period will use

<tt>smp_call_function_single()</tt> to send the CPU an IPI, which

is handled by <tt>sync_rcu_exp_handler()</tt>.

is handled by <tt>rcu_exp_handler()</tt>.



<p>

However, because this is preemptible RCU, <tt>sync_rcu_exp_handler()</tt>

However, because this is preemptible RCU, <tt>rcu_exp_handler()</tt>

can check to see if the CPU is currently running in an RCU read-side

critical section.

If not, the handler can immediately report a quiescent state.
@@ -145,19 +145,18 @@ expedited grace period is shown in the following diagram: 
<p><img src="ExpSchedFlow.svg" alt="ExpSchedFlow.svg" width="55%">



<p>

As with RCU-preempt's <tt>synchronize_rcu_expedited()</tt>,

As with RCU-preempt, RCU-sched's

<tt>synchronize_sched_expedited()</tt> ignores offline and

idle CPUs, again because they are in remotely detectable

quiescent states.

However, the <tt>synchronize_rcu_expedited()</tt> handler

is <tt>sync_sched_exp_handler()</tt>, and because the

However, because the

<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>

leave no trace of their invocation, in general it is not possible to tell

whether or not the current CPU is in an RCU read-side critical section.

The best that <tt>sync_sched_exp_handler()</tt> can do is to check

The best that RCU-sched's <tt>rcu_exp_handler()</tt> can do is to check

for idle, on the off-chance that the CPU went idle while the IPI

was in flight.

If the CPU is idle, then <tt>sync_sched_exp_handler()</tt> reports

If the CPU is idle, then <tt>rcu_exp_handler()</tt> reports

the quiescent state.



<p> Otherwise, the handler forces a future context switch by setting the
@@ -298,19 +297,18 @@ Instead, the task pushing the grace period forward will include the 
idle CPUs in the mask passed to <tt>rcu_report_exp_cpu_mult()</tt>.



<p>

For RCU-sched, there is an additional check for idle in the IPI

handler, <tt>sync_sched_exp_handler()</tt>.

For RCU-sched, there is an additional check:

If the IPI has interrupted the idle loop, then

<tt>sync_sched_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>

<tt>rcu_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>

to report the corresponding quiescent state.



<p>

For RCU-preempt, there is no specific check for idle in the

IPI handler (<tt>sync_rcu_exp_handler()</tt>), but because

IPI handler (<tt>rcu_exp_handler()</tt>), but because

RCU read-side critical sections are not permitted within the

idle loop, if <tt>sync_rcu_exp_handler()</tt> sees that the CPU is within

idle loop, if <tt>rcu_exp_handler()</tt> sees that the CPU is within

RCU read-side critical section, the CPU cannot possibly be idle.

Otherwise, <tt>sync_rcu_exp_handler()</tt> invokes

Otherwise, <tt>rcu_exp_handler()</tt> invokes

<tt>rcu_report_exp_rdp()</tt> to report the corresponding quiescent

state, regardless of whether or not that quiescent state was due to

the CPU being idle.
@@ -625,6 +623,8 @@ checks, but only during the mid-boot dead zone. 
<p>

With this refinement, synchronous grace periods can now be used from

task context pretty much any time during the life of the kernel.

That is, aside from some points in the suspend, hibernate, or shutdown

code path.



<h3><a name="Summary">

Summary</a></h3>

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html

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@@ -485,13 +485,13 @@ section that the grace period must wait on. 
noted by <tt>rcu_node_context_switch()</tt> on the left.

On the other hand, if the CPU takes a scheduler-clock interrupt

while executing in usermode, a quiescent state will be noted by

<tt>rcu_check_callbacks()</tt> on the right.

<tt>rcu_sched_clock_irq()</tt> on the right.

Either way, the passage through a quiescent state will be noted

in a per-CPU variable.



<p>The next time an <tt>RCU_SOFTIRQ</tt> handler executes on

this CPU (for example, after the next scheduler-clock

interrupt), <tt>__rcu_process_callbacks()</tt> will invoke

interrupt), <tt>rcu_core()</tt> will invoke

<tt>rcu_check_quiescent_state()</tt>, which will notice the

recorded quiescent state, and invoke

<tt>rcu_report_qs_rdp()</tt>.
@@ -651,7 +651,7 @@ to end. 
These callbacks are identified by <tt>rcu_advance_cbs()</tt>,

which is usually invoked by <tt>__note_gp_changes()</tt>.

As shown in the diagram below, this invocation can be triggered by

the scheduling-clock interrupt (<tt>rcu_check_callbacks()</tt> on

the scheduling-clock interrupt (<tt>rcu_sched_clock_irq()</tt> on

the left) or by idle entry (<tt>rcu_cleanup_after_idle()</tt> on

the right, but only for kernels build with

<tt>CONFIG_RCU_FAST_NO_HZ=y</tt>).

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