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Date: Sun, 26 Feb 2006 10:06:24 -0700
From: Scott Long <scottl@samsco.org>
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To: Wei Lei <weilei84@gmail.com>
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Cc: freebsd-fs@freebsd.org, Manish Rai Jain <manishrjain@gmail.com>
Subject: Re: Doubt about ntfs code: lockmgr with LK_INTERLOCK flag
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Wei Lei wrote:

> Hi list,
> 
> Nice to be here =)
> 
> I am new here so just hope my first post doesn't mismatch the topic.
> 
> I have a doubt when understanding the ntfs code with freebsd 5.4:
> ntfs_subr.c, function ntfs_ntget().
> I understand that lockmgr() here is trying to get an exclusive lock and
> release the mutex ip->i_interlock,
> so that the protection can expand from use_count to the whole ntnode.
> 
> But what's the significance of having such a mechanism? What could possibly
> happen if I do a
> mtx_unlock first then acquire the exclusive lock?
> 

Doing a mtx_unlock() could trigger your thread to get preempted by a
higher priority thread that could then come in and modify the i_usecount
field.

Scott