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Date: Thu, 12 Feb 2015 22:27:46 +0100 From: Oleg Nesterov <oleg@...hat.com> To: Nicholas Mc Guire <der.herr@...r.at> Cc: Davidlohr Bueso <dave@...olabs.net>, paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org, waiman.long@...com, peterz@...radead.org, raghavendra.kt@...ux.vnet.ibm.com Subject: Re: BUG: spinlock bad magic on CPU#0, migration/0/9 Nicholas, sorry, I sent the patch but forgot to CC you. See https://lkml.org/lkml/2015/2/12/587 And please note that "completion" was specially designed to guarantee that complete() can't play with this memory after wait_for_completion/etc returns. On 02/12, Oleg Nesterov wrote: > > 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 majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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