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

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Date:	Thu, 23 Jul 2015 00:33:15 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Spencer Baugh <sbaugh@...ern.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	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>
CC:	Joern Engel <joern@...estorage.com>,
	Spencer Baugh <Spencer.baugh@...estorage.com>
Subject: RE: [PATCH] target: Drop iSCSI use of mutex around max_cmd_sn
 increment

> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
> owner@...r.kernel.org] On Behalf Of Spencer Baugh
> Sent: Wednesday, July 22, 2015 5:08 PM
> Subject: [PATCH] target: Drop iSCSI use of mutex around max_cmd_sn
> increment
...
> diff --git a/drivers/target/iscsi/iscsi_target_configfs.c
> b/drivers/target/iscsi/iscsi_target_configfs.c
> index c1898c8..29d5930 100644
> --- a/drivers/target/iscsi/iscsi_target_configfs.c
> +++ b/drivers/target/iscsi/iscsi_target_configfs.c
> @@ -706,8 +706,8 @@ static ssize_t lio_target_nacl_show_info(
>  		rb += sprintf(page+rb, " 0x%08x   0x%08x   0x%08x   0x%08x"
>  				"   0x%08x   0x%08x\n",
>  			sess->cmdsn_window,
> -			(sess->max_cmd_sn - sess->exp_cmd_sn) + 1,
> -			sess->exp_cmd_sn, sess->max_cmd_sn,
> +			((u32) atomic_read(&sess->max_cmd_sn) - sess-
> >exp_cmd_sn) + 1,
> +			sess->exp_cmd_sn, (u32) atomic_read(&sess->max_cmd_sn),
>  			sess->init_task_tag, sess->targ_xfer_tag);
>  		rb += sprintf(page+rb, "----------------------[iSCSI"
>  				" Connections]-------------------------\n");

Two calls to atomic_read could pick up different values; 
calling it once and using the same value twice would ensure
the arguments are consistent with each other.

> diff --git a/drivers/target/iscsi/iscsi_target_device.c
> b/drivers/target/iscsi/iscsi_target_device.c
> index 5fabcd3..a526904 100644
> --- a/drivers/target/iscsi/iscsi_target_device.c
> +++ b/drivers/target/iscsi/iscsi_target_device.c
...
> @@ -57,9 +57,7 @@ void iscsit_increment_maxcmdsn(struct iscsi_cmd *cmd,
> struct iscsi_session *sess
> 
>  	cmd->maxcmdsn_inc = 1;
> 
> -	mutex_lock(&sess->cmdsn_mutex);
> -	sess->max_cmd_sn += 1;
> -	pr_debug("Updated MaxCmdSN to 0x%08x\n", sess->max_cmd_sn);
> -	mutex_unlock(&sess->cmdsn_mutex);
> +	atomic_inc(&sess->max_cmd_sn);
> +	pr_debug("Updated MaxCmdSN to 0x%08x\n", atomic_read(&sess-
> >max_cmd_sn));
>  }
>  EXPORT_SYMBOL(iscsit_increment_maxcmdsn);

If there is another change before the atomic_read, this would
print a value unrelated to the increment done by this thread. 
atomic_inc_return would provide the appropriate value to print.

---
Robert Elliott, HP Server Storage
--
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