linux-kernel - Re: [PATCH v3] target: Drop iSCSI use of mutex around max_cmd

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Date:	Sun, 02 Aug 2015 23:21:47 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Spencer Baugh <spencer.baugh@...estorage.com>
Cc:	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	Spencer Baugh <sbaugh@...ern.com>,
	Sagi Grimberg <sagig@...lanox.com>,
	Christoph Hellwig <hch@....de>,
	Bart Van Assche <bart.vanassche@...disk.com>,
	Alexei Potashnik <alexei@...estorage.com>,
	Andy Grover <agrover@...hat.com>,
	Christophe Vu-Brugier <cvubrugier@...tmail.fm>,
	Roland Dreier <roland@...estorage.com>,
	Joern Engel <joern@...fs.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	James Bottomley <JBottomley@...n.com>,
	Evgenii Lepikhin <johnlepikhin@...il.com>,
	Hannes Reinecke <hare@...e.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	"open list:TARGET SUBSYSTEM" <linux-scsi@...r.kernel.org>,
	"open list:TARGET SUBSYSTEM" <target-devel@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	Joern Engel <joern@...estorage.com>
Subject: Re: [PATCH v3] target: Drop iSCSI use of mutex around max_cmd_sn
 increment

On Thu, 2015-07-23 at 14:53 -0700, Spencer Baugh wrote:
> From: Roland Dreier <roland@...estorage.com>
> 
> In a performance profile, taking a mutex in iscsit_increment_maxcmdsn()
> shows up very high.  However taking a mutex around "sess->max_cmd_sn += 1"
> seems pretty silly: we're not serializing against other contexts in
> any useful way.
> 
> I did a quick audit and there don't appear to be any other places that
> use max_cmd_sn within the mutex more than once, so this lock can't be
> providing any useful serialization.
> 
> v2: Get correct values for logging
> v3: Fix whitespace damage
> 
> Signed-off-by: Roland Dreier <roland@...estorage.com>
> Signed-off-by: Spencer Baugh <sbaugh@...ern.com>
> ---
>  drivers/target/iscsi/iscsi_target.c          | 18 +++++++++---------
>  drivers/target/iscsi/iscsi_target_configfs.c |  6 ++++--
>  drivers/target/iscsi/iscsi_target_device.c   |  7 ++-----
>  drivers/target/iscsi/iscsi_target_login.c    |  2 +-
>  drivers/target/iscsi/iscsi_target_nego.c     |  9 +++------
>  drivers/target/iscsi/iscsi_target_tmr.c      |  2 +-
>  drivers/target/iscsi/iscsi_target_util.c     |  7 ++++---
>  include/target/iscsi/iscsi_target_core.h     |  2 +-
>  8 files changed, 25 insertions(+), 28 deletions(-)
> 

A nice optimization for iscsi-target + ib_isert on this patch.

Applied to target-pending/for-next. 

Thanks Roland & Co.

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