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    SubjectRe: [patch part-II V2 09/13] x86/entry/common: Split hardirq tracing into lockdep and ftrace parts
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    On 3/8/20 11:24 PM, Thomas Gleixner wrote:
    > 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(-)


    Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>

    alex.


    > --- 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();
    > }
    >
    >

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