• Steven Rostedt (VMware)'s avatar
    rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work · 03ecd3f4
    Steven Rostedt (VMware) authored
    Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when
    it needs to use rcu_read_lock() and friends. This is because tracing can
    happen as RCU is about to enter user space, or about to go idle, and RCU
    does not watch for RCU read side critical sections as it makes the
    There is a small location within the RCU infrastructure that rcu_irq_enter()
    itself will not work. If tracing were to occur in that section it will break
    if it tries to use rcu_irq_enter().
    Originally, this happens with the stack_tracer, because it will call
    save_stack_trace when it encounters stack usage that is greater than any
    stack usage it had encountered previously. There was a case where that
    happened in the RCU section where rcu_irq_enter() did not work, and lockdep
    complained loudly about it. To fix it, stack tracing added a call to be
    disabled and RCU would disable stack tracing during the critical section
    that rcu_irq_enter() was inoperable. This solution worked, but there are
    other cases that use rcu_irq_enter() and it would be a good idea to let RCU
    give a way to let others know that rcu_irq_enter() will not work. For
    example, in trace events.
    Another helpful aspect of this change is that it also moves the per cpu
    variable called in the RCU critical section into a cache locale along with
    other RCU per cpu variables used in that same location.
    I'm keeping the stack_trace_disable() code, as that still could be used in
    the future by places that really need to disable it. And since it's only a
    static inline, it wont take up any kernel text if it is not used.
    Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.homeAcked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
trace_stack.c 11 KB