Re: [PATCH 16/36] rcu: Fix rcu_idle_exit()

, Arnd Bergmann [off-list ref], ulli.kroll@googlemail.com, vgupta@kernel.org, linux-clk@vger.kernel.org, josh@joshtriplett.org, rostedt@goodmis.org, rcu@vger.kernel.org, bp@alien8.de, bcain@quicinc.com, tsbogend@alpha.franken.de, linux-parisc@vger.kernel.org, sudeep.holla@arm.com, shawnguo@kernel.org, davem@davemloft.net, dalias@libc.org, tony@atomide.com, amakhalov@vmware.com, bjorn.andersson@linaro.org, hpa@zytor.com, sparclinux@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-riscv@lists.infradead.org, anton.ivanov@cambridgegreys.com, jonas@southpole.se, yury.norov@gmail.com, richard@nod.at, x86@kernel.org, linux@armlinux.org.uk, mingo@redhat.com, aou@eecs.berkeley.edu, hca@linux.ibm.com, stefan.kristiansson@saunalahti.fi, openrisc@lists.librecores.org, paul.walmsley@sifive.com, linux-tegra@vger.kernel.org, namhyung@kernel.org, andriy.shevchenko@linux.intel.com, jpoimboe@kernel.org, jgross@suse.com, monstr@monstr.eu, linux-mips@vger.kernel.org, palmer@dabbelt.com, anup@brain
 fault.org, ink@jurassic.park.msu.ru, johannes@sipsolutions.net, linuxppc-dev@lists.ozlabs.org
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Sender: "Linuxppc-dev" [off-list ref]

On Wed, Jun 08, 2022 at 04:27:39PM +0200, Peter Zijlstra wrote:

Current rcu_idle_exit() is terminally broken because it uses
local_irq_{save,restore}(), which are traced which uses RCU.

However, now that all the callers are sure to have IRQs disabled, we
can remove these calls.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>

We have some fun conflicts between this series and Frederic's context-tracking
series.  But it looks like these can be resolved by:

1.	A patch on top of Frederic's series that provides the old rcu_*()
	names for the functions now prefixed with ct_*() such as
	ct_idle_exit().

2.	Another patch on top of Frederic's series that takes the
	changes remaining from this patch, shown below.  Frederic's
	series uses raw_local_irq_save() and raw_local_irq_restore(),
	which can then be removed.

Or is there a better way to do this?

							Thanx, Paul

------------------------------------------------------------------------

commit f64cee8c159e9863a74594efe3d33fb513a6a7b5
Author: Peter Zijlstra [off-list ref]
Date:   Tue Jun 14 17:24:43 2022 -0700

    context_tracking: Interrupts always disabled for ct_idle_exit()
    
    Now that the idle-loop cleanups have ensured that rcu_idle_exit() is
    always invoked with interrupts disabled, remove the interrupt disabling
    in favor of a debug check.
    
    Signed-off-by: Peter Zijlstra [off-list ref]
    Cc: Frederic Weisbecker [off-list ref]
    Signed-off-by: Paul E. McKenney [off-list ref]

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 1da44803fd319..99310cf5b0254 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c

@@ -332,11 +332,8 @@ EXPORT_SYMBOL_GPL(ct_idle_enter);
  */
 void noinstr ct_idle_exit(void)
 {
-	unsigned long flags;
-
-	raw_local_irq_save(flags);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
 	ct_kernel_enter(false, RCU_DYNTICKS_IDX - CONTEXT_IDLE);
-	raw_local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(ct_idle_exit);