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

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Message-ID: <20120516202215.GO22985@linux.intel.com>
Date:	Wed, 16 May 2012 16:22:15 -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 03:54:06PM -0400, chetan loke wrote:
> I thought that 'scsi_ram_device_thread' has a
> 'wait_event_interruptible' call if it sees that the list is empty. I
> don't see that call that's why the confusion. Either queuecmd or
> kthread will get the lock and so why keep the kthread spinning in that
> while loop if there's nothing to do?

It doesn't spin, it sleeps:

+               spin_lock_irqsave(host->host_lock, flags);
+               if (list_empty(&ram_device->commands)) {
+                       set_current_state(TASK_INTERRUPTIBLE);
+                       spin_unlock_irqrestore(host->host_lock, flags);
+                       schedule();
+                       continue;
+               }

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