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[Andy Whitcroft <apw@xxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu Trusty)
       Status: Fix Committed => Fix Released

** Tags removed: kernel-bug-break-fix

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https://bugs.launchpad.net/bugs/1410741

Title:
  VDSO clock function may be miss-optimised leading to hangs on SMP
  systems

Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Trusty:
  Fix Released
Status in linux source package in Utopic:
  Fix Released
Status in linux source package in Vivid:
  Fix Released

Bug description:
  commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba
  Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
  Date:   Sun Dec 21 08:57:46 2014 -0800

      x86, vdso: Use asm volatile in __getcpu

      In Linux 3.18 and below, GCC hoists the lsl instructions in the
      pvclock code all the way to the beginning of __vdso_clock_gettime,
      slowing the non-paravirt case significantly.  For unknown reasons,
      presumably related to the removal of a branch, the performance issue
      is gone as of

      e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu

      but I don't trust GCC enough to expect the problem to stay fixed.

      There should be no correctness issue, because the __getcpu calls in
      __vdso_vlock_gettime were never necessary in the first place.

      Note to stable maintainers: In 3.18 and below, depending on
      configuration, gcc 4.9.2 generates code like this:

           9c3:       44 0f 03 e8             lsl    %ax,%r13d
           9c7:       45 89 eb                mov    %r13d,%r11d
           9ca:       0f 03 d8                lsl    %ax,%ebx

      This patch won't apply as is to any released kernel, but I'll send a
      trivial backported version if needed.

      Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
      Cc: stable@xxxxxxxxxxxxxxx # 3.8+
      Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
      Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
      Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

  ===
  break-fix: 51c19b4f5927f5a646e93d69f73c7e89ea14e737 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba

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