[patch part-II V2 09/13] x86/entry/common: Split hardirq tracing into lockdep and ftrace parts

Thomas Gleixner
2020-03-08T23:25:46Z

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();
}