linux-kernel - Re: Memory barrier needed with wake_up

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Message-ID: <20160906150547.GG2794@worktop>
Date:   Tue, 6 Sep 2016 17:05:47 +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>,
        Will Deacon <will.deacon@....com>
Subject: Re: Memory barrier needed with wake_up_process()?

On Tue, Sep 06, 2016 at 10:46:55AM -0400, Alan Stern wrote:

Not knowing where INFO() goes, you should use trace_printk() not
printk(), as the former is strictly per cpu, while the latter is
globally serialized and can hide all these problems.

> 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;
>