linux-kernel - Re: Memory barrier needed with wake_up

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Date:   Mon, 5 Sep 2016 10:08:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Felipe Balbi <felipe.balbi@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Memory barrier needed with wake_up_process()?

On Sat, Sep 03, 2016 at 10:16:31AM -0400, Alan Stern wrote:

> > Sorry, but that is horrible code. A barrier cannot ensure writes are
> > 'complete', at best they can ensure order between writes (or reads
> > etc..).
> 
> The code is better than the comment.  What I really meant was that the 
> write of bh->state needs to be visible to the thread after it wakes up 
> (or after it checks the wakeup condition and skips going to sleep).

Yeah, I got that.

> > Also, looking at that thing, that common->thread_wakeup_needed variable
> > is 100% redundant. All sleep_thread() invocations are inside a loop of
> > sorts and basically wait for other conditions to become true.
> > 
> > For example:
> > 
> > 	while (bh->state != BUF_STATE_EMPTY) {
> > 		rc = sleep_thread(common, false);
> > 		if (rc)
> > 			return rc;
> > 	}
> > 
> > All you care about there is bh->state, _not_
> > common->thread_wakeup_needed.
> 
> You know, I never went through and verified that _all_ the invocations 
> of sleep_thread() are like that. 

Well, thing is, they're all inside a loop which checks other conditions
for forward progress. Therefore the loop inside sleep_thread() is
pointless. Even if you were to return early, you'd simply loop in the
outer loop and go back to sleep again.

> In fact, I wrote the sleep/wakeup 
> routines _before_ the rest of the code, and I didn't know in advance 
> exactly how they were going to be called.

Still seems strange to me, why not use wait-queues for the first cut?

Only if you find a performance issue with wait-queues, which cannot be
fixed in the wait-queue proper, then do you do custom thingies.

Starting with a custom sleeper, just doesn't make sense to me.

> > That said, I cannot spot an obvious fail, but the code can certainly use
> > help.
> 
> The problem may be that when the thread wakes up (or skips going to 
> sleep), it needs to see more than just bh->state.  Those other values 
> it needs are not written by the same CPU that calls wakeup_thread(), 
> and so to ensure that they are visible that smp_wmb() really ought to 
> be smp_mb() (and correspondingly in the thread.  That's what Felipe has 
> been testing.

So you're saying something like:


	CPU0		CPU1		CPU2

	X = 1				sleep_thread()
			wakeup_thread()
					r = X

But how does CPU1 know to do the wakeup? That is, how are CPU0 and CPU1
coupled.