Doubt about ntfs code: lockmgr with LK_INTERLOCK flag

[Scott Long]

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