On 10/16, Paul E. McKenney wrote: > > On Tue, Oct 16, 2012 at 05:56:23PM +0200, Oleg Nesterov wrote: > > > > > > I believe that you need smp_mb() here. > > > > I don't understand why... > > > > > The wake_up_all()'s memory barriers > > > do not suffice because some other reader might have awakened the writer > > > between this_cpu_dec() and wake_up_all(). > > > > But __wake_up(q) takes q->lock? And the same lock is taken by > > prepare_to_wait(), so how can the writer miss the result of _dec? > > Suppose that the writer arrives and sees that the value of the counter > is zero,
after synchronize_sched(). So there are no readers (but perhaps there are brw_end_read's in flight which already decremented read_ctr) > and thus never sleeps, and so is also not awakened? and why do we need wakeup in this case? > > > void brw_end_read(struct brw_mutex *brw) > > > { > > > if (unlikely(atomic_read(&brw->write_ctr))) { > > > smp_mb(); > > > this_cpu_dec(*brw->read_ctr); > > > wake_up_all(&brw->write_waitq); > > > > Hmm... still can't understand. > > > > It seems that this mb() is needed to ensure that brw_end_read() can't > > miss write_ctr != 0. > > > > But we do not care unless the writer already does wait_event(). And > > before it does wait_event() it calls synchronize_sched() after it sets > > write_ctr != 0. Doesn't this mean that after that any preempt-disabled > > section must see write_ctr != 0 ? > > > > This code actually checks write_ctr after preempt_disable + enable, > > but I think this doesn't matter? > > > > Paul, most probably I misunderstood you. Could you spell please? > > Let me try outlining the sequence of events that I am worried about... > > 1. Task A invokes brw_start_read(). There is no writer, so it > takes the fastpath. > > 2. Task B invokes brw_start_write(), atomically increments > &brw->write_ctr, and executes synchronize_sched(). > > 3. Task A invokes brw_end_read() and does this_cpu_dec(). OK. And to simplify this discussion, suppose that A invoked brw_start_read() on CPU_0 and thus incremented read_ctr[0], and then it migrates to CPU_1 and brw_end_read() uses read_ctr[1]. My understanding was, brw_start_write() must see read_ctr[0] == 1 after synchronize_sched(). > 4. Task B invokes wait_event(), which invokes brw_read_ctr() > and sees the result as zero. So my understanding is completely wrong? I thought that after synchronize_sched() we should see the result of any operation which were done inside the preempt-disable section. No? Hmm. Suppose that we have long A = B = STOP = 0, and void func(void) { preempt_disable(); if (!STOP) { A = 1; B = 1; } preempt_enable(); } Now, you are saying that this code STOP = 1; synchronize_sched(); BUG_ON(A != B); is not correct? (yes, yes, this example is not very good). The comment above synchronize_sched() says: return ... after all currently executing rcu-sched read-side critical sections have completed. But if this code is wrong, then what "completed" actually means? I thought that it also means "all memory operations have completed", but this is not true? 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/