linux-kernel - Re: SCSI RAM driver ported to 3.3 kernel for file system and I/O testing

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Date:	Wed, 16 May 2012 15:43:32 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	chetan loke <loke.chetan@...il.com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-scsi@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: SCSI RAM driver ported to 3.3 kernel for file system and I/O
 testing

On Wed, May 16, 2012 at 03:37:56PM -0400, chetan loke wrote:
> On Wed, May 16, 2012 at 3:35 PM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> > On Wed, May 16, 2012 at 03:31:55PM -0400, chetan loke wrote:
> >> > +               if (list_empty(&ram_device->commands))
> >> > +                       wake_up_process(ram_device->thread);
> >>
> >> Didn't look in detail but if the queue is empty then why would you
> >> want to wake up the kthread? What if you just wake_up after
> >> list_add_tail below?
> >
> > If the list is non-empty, then the kthread has already been woken up
> > and doesn't need to be woken again.
> 
> Sorry, not able to follow. wait_even_interruptible will put kthread to
> sleep. So how will it be already awake?

Consider the following:

CPU 0			CPU 1
->queuecommand
lock
wakes kthread
queues command 1
unlock

->queuecommand
lock
			kthread wakes
			lock
queues command 2
unlock
			dequeues command 1
			dequeues command 2
			unlock


See?  No need to wake the kthread *if there's already something on the
queue*, because you know it was already woken by whoever put the first
command on the queue.
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