linux-kernel - Re: [RFC v3 01/15] scsi: Drop struct Scsi_Host->host_lock usage in scsi_dispatch

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Message-ID: <20100927141923.GC8473@schmichrtp.mainz.de.ibm.com>
Date:	Mon, 27 Sep 2010 16:19:23 +0200
From:	Christof Schmitt <christof.schmitt@...ibm.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Brian King <brking@...ux.vnet.ibm.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Mike Christie <michaelc@...wisc.edu>,
	James Smart <james.smart@...lex.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>,
	Joe Eykholt <jeykholt@...co.com>,
	Christoph Hellwig <hch@....de>,
	MPTFusionLinux <DL-MPTFusionLinux@....com>,
	"eata.c maintainer" <dario.ballabio@...ind.it>
Subject: Re: [RFC v3 01/15] scsi: Drop struct Scsi_Host->host_lock usage in
 scsi_dispatch_cmd()

On Fri, Sep 24, 2010 at 01:44:00PM -0700, Nicholas A. Bellinger wrote:
> On Fri, 2010-09-24 at 08:41 -0500, Brian King wrote:
> > On 09/23/2010 06:37 PM, Nicholas A. Bellinger wrote:
> > > @@ -651,7 +655,6 @@ static inline void scsi_cmd_get_serial(struct Scsi_Host *host, struct scsi_cmnd
> > >  int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
> > >  {
> > >  	struct Scsi_Host *host = cmd->device->host;
> > > -	unsigned long flags = 0;
> > >  	unsigned long timeout;
> > >  	int rtn = 0;
> > > 
> > > @@ -736,15 +739,11 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
> > >  		scsi_done(cmd);
> > >  		goto out;
> > >  	}
> > > -
> > > -	spin_lock_irqsave(host->host_lock, flags);
> > >  	/*
> > > -	 * AK: unlikely race here: for some reason the timer could
> > > -	 * expire before the serial number is set up below.
> > > -	 *
> > > -	 * TODO: kill serial or move to blk layer
> > > +	 * Note that scsi_cmd_get_serial() used to be called here, but
> > > +	 * now we expect the legacy SCSI LLDs that actually need this
> > > +	 * to call it directly within their SHT->queuecommand() caller.
> > >  	 */
> > > -	scsi_cmd_get_serial(host, cmd); 
> > > 
> > >  	if (unlikely(host->shost_state == SHOST_DEL)) {
> > >  		cmd->result = (DID_NO_CONNECT << 16);
> > > @@ -753,7 +752,7 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
> > >  		trace_scsi_dispatch_cmd_start(cmd);
> > >  		rtn = host->hostt->queuecommand(cmd, scsi_done);
> > >  	}
> > > -	spin_unlock_irqrestore(host->host_lock, flags);
> > > +
> > >  	if (rtn) {
> > >  		trace_scsi_dispatch_cmd_error(cmd, rtn);
> > >  		if (rtn != SCSI_MLQUEUE_DEVICE_BUSY &&
> > 
> > Are you planning a future revision that moves the acquiring of the host lock
> > into the LLDD's queuecommand for all the other drivers you don't currently
> > touch in this patch set?
> > 
> 
> Hi Brian,
> 
> I was under the impression that this would be unnecessary for the vast
> majority of existing LLD drivers, but if you are aware of specific LLDs
> that would still need the struct Scsi_Host->host_lock held in their
> SHT->queuecommand() for whaterver reason please let me know and I would
> be happy to include this into an RFCv4.
> 
> Thanks for your comments!

zfcp relies on having the interrupts disabled when calling
queuecommand. Without the spin_lock_irqsave in scsi_dispatch_cmd, the
locking in zfcp_fsf_send_fcp_command_task has to be changed from
spin_lock(&qdio->req_q_lock) to spin_lock_irqsave. It is a simple
change, but other drivers might have similar requirements.

Christof
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