linux-kernel - Re: [GIT pull] locking/urgent for v5.10-rc6

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Message-ID: <20201201181506.GM3092@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Dec 2020 19:15:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] locking/urgent for v5.10-rc6

On Tue, Dec 01, 2020 at 03:55:19PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2020 at 06:46:44AM -0800, Paul E. McKenney wrote:
> 
> > > So after having talked to Sven a bit, the thing that is happening, is
> > > that this is the one place where we take interrupts with RCU being
> > > disabled. Normally RCU is watching and all is well, except during idle.
> > 
> > Isn't interrupt entry supposed to invoke rcu_irq_enter() at some point?
> > Or did this fall victim to recent optimizations?
> 
> It does, but the problem is that s390 is still using

I might've been too quick there, I can't actually seem to find where
s390 does rcu_irq_enter()/exit().

Also, I'm thinking the below might just about solve the current problem.
The next problem would then be it calling TRACE_IRQS_ON after it did
rcu_irq_exit()... :/


---
diff --git a/arch/s390/include/asm/irq.h b/arch/s390/include/asm/irq.h
index 9f75d67b8c20..24d3dd482df7 100644
--- a/arch/s390/include/asm/irq.h
+++ b/arch/s390/include/asm/irq.h
@@ -113,6 +113,8 @@ void irq_subclass_unregister(enum irq_subclass subclass);
 
 #define irq_canonicalize(irq)  (irq)
 
+extern __visible void ext_do_IRQ(struct pt_regs *regs, unsigned long vector);
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_IRQ_H */
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 26bb0603c5a1..b8e89b685038 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -976,16 +976,10 @@ ENTRY(ext_int_handler)
 	xc	__PT_FLAGS(8,%r11),__PT_FLAGS(%r11)
 	TSTMSK	__LC_CPU_FLAGS,_CIF_IGNORE_IRQ
 	jo	.Lio_restore
-#if IS_ENABLED(CONFIG_TRACE_IRQFLAGS)
-	tmhh	%r8,0x300
-	jz	1f
-	TRACE_IRQS_OFF
-1:
-#endif
 	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
 	lgr	%r2,%r11		# pass pointer to pt_regs
 	lghi	%r3,EXT_INTERRUPT
-	brasl	%r14,do_IRQ
+	brasl	%r14,ext_do_IRQ
 	j	.Lio_return
 ENDPROC(ext_int_handler)
 
diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
index 3514420f0259..f4a29114e9fd 100644
--- a/arch/s390/kernel/irq.c
+++ b/arch/s390/kernel/irq.c
@@ -329,3 +329,23 @@ void irq_subclass_unregister(enum irq_subclass subclass)
 	spin_unlock(&irq_subclass_lock);
 }
 EXPORT_SYMBOL(irq_subclass_unregister);
+
+noinstr void ext_do_IRQ(struct pt_regs *regs, unsigned long vector)
+{
+	bool rcu = false;
+
+	lockdep_hardirqs_off(CALLER_ADDR0);
+	if (!IS_ENABLED(CONFIG_TINY_RCU) && is_idle_task(current)) {
+		rcu_irq_enter();
+		rcu = true;
+	}
+	/* instrumentation_begin(); */
+
+	trace_hardirqs_off_finish();
+
+	do_IRQ(regs, vector);
+
+	/* instrumentation_end(); */
+	if (rcu)
+		rcu_irq_exit();
+}