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[Chris J Arges <1226726@xxxxxxxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu Precise)
   Importance: Medium => High

** Changed in: linux (Ubuntu Raring)
   Importance: Medium => High

** Changed in: linux (Ubuntu Quantal)
   Importance: Medium => High

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https://bugs.launchpad.net/bugs/1226726

Title:
  dentry_reset_mounted walks entire mount list holding vfsmount write
  lock

Status in “linux” package in Ubuntu:
  Fix Released
Status in “linux” source package in Precise:
  In Progress
Status in “linux” source package in Quantal:
  In Progress
Status in “linux” source package in Raring:
  In Progress
Status in “linux” source package in Saucy:
  Fix Released

Bug description:
  Whenever one enters a network namespace via "ip netns exec foobar
  somecommand" there is a mount done of the appropriate device on /sys
  since "somecommand" needs to see namespace specific versions of /sys
  directories. When the ip process exits these mounts need to be torn
  down, and that requires a global write lock for vfsmount_lock (this is
  a single writer multiple reader lock). This has serious performance
  implications when the number of name spaces increase.

  The commit 84d17192 addresses this issue, and it is clear by running
  the attached testcase that it fixes performance issues when dealing
  with large numbers of namespaces. I've included a graph with the
  differences in performance between this fix and its parent commit to
  show the the improve in performance. The x-axis represents the number
  of namespaces and the y-axis is execution time in ms. After applying
  the patch the performance delays are not exponentially increasing.

  This affects 3.2/3.5/3.8 series kernels, as it was fixed in 3.10.

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