linux-kernel - Re: [PATCH scsi] Add NULL checking of return value from scsi_cmd_to

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Message-ID: <1348472104.2467.8.camel@dabdike>
Date:	Mon, 24 Sep 2012 11:35:04 +0400
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-scsi@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Martin K. Petersen" <mkp@....net>
Subject: Re: [PATCH scsi] Add NULL checking of return value from
 scsi_cmd_to_driver()

On Mon, 2012-09-24 at 15:03 +0800, Li Zhong wrote:
> On Mon, 2012-09-24 at 09:44 +0400, James Bottomley wrote:
> > On Mon, 2012-09-24 at 13:30 +0800, Li Zhong wrote:
> > > Just noticed that after commit 919f797, it is possible that 
> > > scsi_cmd_to_driver() returns NULL. This patch adds the NULL checking for drv 
> > > returned from the above function. 
> > > 
> > > Maybe it is not possible at run time, but from the code itself, we'd better
> > > have this check?  
> > 
> > There's not much point having a check that never trips, unless it's an
> > assert, in which case a NULL deref does that.  All it does is add
> > pointless instructions to the critical path.  only REQ_TYPE_BLOCK_PC
> > commands can be submitted without a driver, so the check above would
> > seem to preclude that.
> 
> Hi James, 
> 
> Thank you, it sounds reasonable to me. Let's drop it. 

Well, there is another thing you might do: The path length of
scsi_cmd_to_driver() increased a lot thanks to 18a4d0a22ed6 it might be
worth getting it back to what it was (this looks to be doable with the
same != REQ_TYPE_BLOCK_PC test in the error handler.  Plus, I think it
fixes a bug where you get different behaviours from REQ_TYPE_BLOCK_PC
commands when a driver is and isn't attached (I've cc'd Martin to see
what he thinks).

James


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