On 02/12, Nicholas Mc Guire wrote: > > On Thu, 12 Feb 2015, Oleg Nesterov wrote: > > > No, sorry, only the 2nd one. > > > > > Unless at least document how > > > you can use these helpers. > > > > > > Consider this code: > > > > > > void xxx(void) > > > { > > > struct completion c; > > > > > > init_completion(&c); > > > > > > expose_this_completion(&c); > > > > > > while (!completion_done(&c) > > > schedule_timeout_uninterruptible(1); > > But that would not break due to the change - even if completion_done() had a > problem - complete_done() is not consuming x->done it is only checking it?
Nicholas, looks like you didn't read the text below: > > > Before that change this code was correct, now it is not. Hmm and note that > > > this is what stop_machine_from_inactive_cpu() does although I do not know > > > if this is related or not. > > > > > > Because completion_done() can now race with complete(), the final > > > spin_unlock() can write to the memory after it was freed/reused. In this > > > case it can write to the stack after return. Or I misunderstood you. > > bool completion_done(struct completion *x) > > { > > - return !!ACCESS_ONCE(x->done); > > + if (!READ_ONCE(x->done)) > > + return false; > > + > > + smp_rmb(); > > + spin_unlock_wait(&x->wait.lock); > > + return true; > > what would be the sense of the spin_unlock_wait here ? > all you are interested in here is the state of x->done No. Please see above. > regarding the smp_rmb() where would the counterpart to that be ? to avoid the reordering, we should not read ->wait.lock before ->done. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/