linux-kernel - Re: Memory barrier needed with wake_up

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Message-ID: <Pine.LNX.4.44L0.1609061043420.1951-100000@iolanthe.rowland.org>
Date:   Tue, 6 Sep 2016 10:46:55 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Peter Zijlstra <peterz@...radead.org>
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>,
        Will Deacon <will.deacon@....com>
Subject: Re: Memory barrier needed with wake_up_process()?

On Tue, 6 Sep 2016, Peter Zijlstra wrote:

> On Tue, Sep 06, 2016 at 01:49:37PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 06, 2016 at 02:43:39PM +0300, Felipe Balbi wrote:
> 
> > > My fear now, however, is that changing smp_[rw]mb() to smp_mb() just
> > > adds extra overhead which makes the problem much, much less likely to
> > > happen. Does that sound plausible to you?
> > 
> > I did consider that, but I've not sufficiently grokked the code to rule
> > out actual fail. So let me stare at this a bit more.
> 
> OK, so I'm really not seeing it, we've got:
> 
> while (bh->state != FULL) {
>         for (;;) {
>                 set_current_state(INTERRUPTIBLE); /* MB after */
>                 if (signal_pending(current))
>                         return -EINTR;
>                 if (common->thread_wakeup_needed)
>                         break;
>                 schedule(); /* MB */
>         }
>         __set_current_state(RUNNING);
>         common->thread_wakeup_needed = 0;
>         smp_rmb(); /* NOP */
> }
> 
> 
> VS.
> 
> 
> spin_lock(&common->lock); /* MB */
> bh->state = FULL;
> smp_wmb(); /* NOP */
> common->thread_wakeup_needed = 1;
> wake_up_process(common->thread_task); /* MB before */
> spin_unlock(&common->lock);
> 
> 
> 
> (the MB annotations specific to x86, not true in general)
> 
> 
> If we observe thread_wakeup_needed, we must also observe bh->state.
> 
> And the sleep/wakeup ordering is also correct, we either see
> thread_wakeup_needed and continue, or we see task->state == RUNNING
> (from the wakeup) and NO-OP schedule(). The MB from set_current_statE()
> then matches with the MB from wake_up_process() to ensure we must see
> thead_wakeup_needed.
> 
> Or, we go sleep, and get woken up, at which point the same happens.
> Since the waking CPU gets the task back on its RQ the happens-before
> chain includes the waking CPUs state along with the state of the task
> itself before it went to sleep.
> 
> At which point we're back where we started, once we see
> thread_wakeup_needed we must then also see bh->state (and all state
> prior to that on the waking CPU).
> 
> 
> 
> There's enough cruft in the while-sleep loop to force reload bh->state.
> 
> Load/store tearing cannot be a problem because all values are single
> bytes (the variables are multi bytes, but all values used only affect
> the LSB).
> 
> Colour me puzzled.

Felipe, can you please try this patch on an unmodified tree?  If the 
problem still occurs, what shows up in the kernel log?

Alan Stern



Index: usb-4.x/drivers/usb/gadget/function/f_mass_storage.c
===================================================================
--- usb-4.x.orig/drivers/usb/gadget/function/f_mass_storage.c
+++ usb-4.x/drivers/usb/gadget/function/f_mass_storage.c
@@ -485,6 +485,8 @@ static void bulk_out_complete(struct usb
 	spin_lock(&common->lock);
 	bh->outreq_busy = 0;
 	bh->state = BUF_STATE_FULL;
+	if (bh->bulk_out_intended_length == US_BULK_CB_WRAP_LEN)
+		INFO(common, "compl: bh %p state %d\n", bh, bh->state);
 	wakeup_thread(common);
 	spin_unlock(&common->lock);
 }
@@ -2207,6 +2209,7 @@ static int get_next_command(struct fsg_c
 		rc = sleep_thread(common, true);
 		if (rc)
 			return rc;
+		INFO(common, "next: bh %p state %d\n", bh, bh->state);
 	}
 	smp_rmb();
 	rc = fsg_is_set(common) ? received_cbw(common->fsg, bh) : -EIO;