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Message-Id: <20200308222609.825111830@linutronix.de>
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Date:   Sun, 08 Mar 2020 23:24:08 +0100
From:   Thomas Gleixner <tglx@linutronix.de>
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     x86@kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Brian Gerst <brgerst@gmail.com>,
        Juergen Gross <jgross@suse.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>
Subject: [patch part-II V2 09/13] x86/entry/common: Split hardirq tracing into lockdep and ftrace parts
References: <20200308222359.370649591@linutronix.de>
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trace_hardirqs_off() is in fact a tracepoint which can be utilized by BPF,
which is unsafe before calling enter_from_user_mode(), which in turn
invokes context tracking. trace_hardirqs_off() also invokes
lockdep_hardirqs_off() under the hood.

OTOH lockdep needs to know about the interrupts disabled state before
enter_from_user_mode(). lockdep_hardirqs_off() is safe to call at this
point.

Split it so lockdep knows about the state and invoke the tracepoint after
the context is set straight.

Even if the functions attached to a tracepoint would all be safe to be
called in rcuidle having it split up is still giving a performance
advantage because rcu_read_lock_sched() is avoiding the whole dance of:

   scru_read_lock();
   rcu_irq_enter_irqson();
   ...
   rcu_irq_exit_irqson();
   scru_read_unlock();
   
around the tracepoint function invocation just to have the C entry points
of syscalls call enter_from_user_mode() right after the above dance.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: New patch
---
 arch/x86/entry/common.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -60,10 +60,19 @@ static __always_inline void syscall_entr
 {
 	/*
 	 * Usermode is traced as interrupts enabled, but the syscall entry
-	 * mechanisms disable interrupts. Tell the tracer.
+	 * mechanisms disable interrupts. Tell lockdep before calling
+	 * enter_from_user_mode(). This is safe vs. RCU while the
+	 * tracepoint is not.
 	 */
-	trace_hardirqs_off();
+	lockdep_hardirqs_on(CALLER_ADDR0);
+
 	enter_from_user_mode();
+
+	/*
+	 * Tell the tracer about the irq state as well before enabling
+	 * interrupts.
+	 */
+	__trace_hardirqs_off();
 	local_irq_enable();
 }