linux-kernel - __blk_end_request vs blk_end

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [day] [month] [year] [list]

Date:	Sat, 31 Mar 2012 17:13:45 +0530
From:	"Subhash Jadavani" <subhashj@...eaurora.org>
To:	<linux-mmc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Cc:	<linux-arm-msm@...r.kernel.org>, "'Chris Ball'" <cjb@...top.org>
Subject: __blk_end_request vs blk_end_request

Hi,

For completing any block request, MMC block driver is calling:
       spin_lock_irq(queue)
       _blk_end_request()
       spin_unlock_irq(queue)

But if we analyze the sources of latency in kernel using ftrace,
__blk_end_request() function seems to hold a spinlock with interrupts
disabled for up to 6.5 ms sometimes. Attached is ftrace output for this
function.

__blk_end_request() calls couple of functions and ftrace output shows that
blk_update_bidi_request() function is almost taking 6ms. So I was wondering
why cant we use the blk_end_request() rather than __blk_end_request(). Both
function does the same thing except blk_end_request() doesnt take up the
spinlock while calling the blk_update_bidi_request().

Is there any race condition which could occur if we call
blk_update_bidi_request() without queue lock? I looked into
blk_update_bidi_request() function and I mainly updates bio's of a request
and doesn't look to do any manipulation with request queue structure of
block device. There are many block drivers (SCSI, IDE etc ) other than MMC
uses blk_end_request() rather than __blk_end_request().  
Was there any special reason we are using __blk_end_request() in MMC block
driver? If there is no specific reason, I would like to post a patch which
would make MMC driver to use blk_end_request().

Let me know your thoughts on this.

Regards,
Subhash
--
Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

View attachment "ftrace-output.txt" of type "text/plain" (237951 bytes)