On Thu, May 11, 2017 at 03:52:46PM +0200, Petr Mladek wrote: > On Mon 2017-05-08 15:13:22, Steven Rostedt wrote: > > On Mon, 8 May 2017 11:51:08 -0500 > > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > > Another idea would be to figure out a way to stop using RCU in > > > klp_ftrace_handler() altogether. > > > > > > > That may work if rcu_enter_irq() doesn't. But that's how NMIs use rcu. > > I am a bit confused by the above. Does it mean that RCU could not be > used in NMI handlers? Only RCU readers can be used in NMI handlers, that is, rcu_read_lock(), rcu_read_unlock(), rcu_dereference(), and so on. Thanx, Paul > Anyway, a crazy idea is to use the livepatch consistency model instead > of RCU to protect the function stack. The model makes sure that all > tasks, including the idle ones, were not running any patched function > (and their ftrace handlers) at some point. It should be safe > but I am not sure if it is worth it. > > Alternatively, it might be enough to use the probably more lightwight > solution that is used when ftrace handlers are deregistered, I mean: > > /* > * We need to do a hard force of sched synchronization. > * This is because we use preempt_disable() to do RCU, but > * the function tracers can be called where RCU is not watching > * (like before user_exit()). We can not rely on the RCU > * infrastructure to do the synchronization, thus we must do it > * ourselves. > */ > schedule_on_each_cpu(ftrace_sync); > > /* > * When the kernel is preeptive, tasks can be preempted > * while on a ftrace trampoline. Just scheduling a task on > * a CPU is not good enough to flush them. Calling > * synchornize_rcu_tasks() will wait for those tasks to > * execute and either schedule voluntarily or enter user space. > */ > if (IS_ENABLED(CONFIG_PREEMPT)) > synchronize_rcu_tasks(); > > > > Best Regards, > Petr > -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
