On Fri, 7 Apr 2017 09:42:20 -0700 "Paul E. McKenney" <[email protected]> wrote:
> On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote: > > From: "Steven Rostedt (VMware)" <[email protected]> > > > > 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 > > transition. > > > > 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. > > > > Signed-off-by: Steven Rostedt (VMware) <[email protected]> > > Looks better, especially __this_cpu_inc() and __this_cpu_dec(). > > How about rcu_irq_enter_disabled instead of rcu_disabled? We aren't > really disabling all of RCU. ;-) OK, I'll make the update and send a 6.1 (and also a new 7/5)! -- Steve
