• Steven Rostedt's avatar
    tracing: Remove ftrace_preempt_disable/enable · 5168ae50
    Steven Rostedt authored
    The ftrace_preempt_disable/enable functions were to address a
    recursive race caused by the function tracer. The function tracer
    traces all functions which makes it easily susceptible to recursion.
    One area was preempt_enable(). This would call the scheduler and
    the schedulre would call the function tracer and loop.
    (So was it thought).
    The ftrace_preempt_disable/enable was made to protect against recursion
    inside the scheduler by storing the NEED_RESCHED flag. If it was
    set before the ftrace_preempt_disable() it would not call schedule
    on ftrace_preempt_enable(), thinking that if it was set before then
    it would have already scheduled unless it was already in the scheduler.
    This worked fine except in the case of SMP, where another task would set
    the NEED_RESCHED flag for a task on another CPU, and then kick off an
    IPI to trigger it. This could cause the NEED_RESCHED to be saved at
    ftrace_preempt_disable() but the IPI to arrive in the the preempt
    disabled section. The ftrace_preempt_enable() would not call the scheduler
    because the flag was already set before entring the section.
    This bug would cause a missed preemption check and cause lower latencies.
    Investigating further, I found that the recusion caused by the function
    tracer was not due to schedule(), but due to preempt_schedule(). Now
    that preempt_schedule is completely annotated with notrace, the recusion
    no longer is an issue.
    Reported-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
trace_stack.c 7.67 KB