On Wed, Mar 15, 2017 at 05:43:02PM +0100, Oleg Nesterov wrote: > On 03/14, Peter Zijlstra wrote: > > > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -10346,6 +10346,17 @@ void perf_event_free_task(struct task_struct *task) > > continue; > > > > mutex_lock(&ctx->mutex); > > + raw_spin_lock_irq(&ctx->lock); > > + /* > > + * Destroy the task <-> ctx relation and mark the context dead. > > + * > > + * This is important because even though the task hasn't been > > + * exposed yet the context has been (through child_list). > > + */ > > + RCU_INIT_POINTER(task->perf_event_ctxp[ctxn], NULL); > > + WRITE_ONCE(ctx->task, TASK_TOMBSTONE); > > + put_task_struct(task); /* cannot be last */ > > + raw_spin_unlock_irq(&ctx->lock); > > Agreed, this is what I had in mind. Although you know, I spent 3 > hours looking at your patch and I still can't convince myself I am > really sure it closes all races ;)
Ha; yes I know that feeling. I used to have a few sheets of paper filled with diagrams. Sadly I could not find them again. Must've been over eager cleaning my desk at some point. > > OK, I believe this is correct. And iiuc both RCU_INIT_POINTER(NULL) > and put_task_struct() are not strictly necessary? At least until we > add WARN_ON(tsk->usage != 2) before free_task() in copy process(). Right; I just kept the code similar to the other location. I even considered making a helper function to not duplicate, but in the end decided against it. > --------------------------------------------------------------------- > This is off-topic, but to me list_for_each_entry(event->child_list) > in perf_event_release_kernel() looks very confusing and misleading. > And list_first_entry_or_null(), we do not really need NULL if list > is empty, tmp == child should be F even if we use list_first_entry(). > And given that we already have list_is_last(), it would be nice to > add list_is_first() and cleanup perf_event_release_kernel() a bit: > Agreed; its a bit of a weird one. Let me go write proper patches for the things we have so far though.