git: kernel - Rewrite umtx_sleep() and umtx_wakeup()

Matthew Dillon dillon at
Mon Oct 16 11:30:38 PDT 2017

commit afd7f1247cb20d5b03f8ccddb470de6a55afc530
Author: Matthew Dillon <dillon at>
Date:   Mon Oct 16 00:28:11 2017 -0700

    kernel - Rewrite umtx_sleep() and umtx_wakeup()
    * Rewrite umtx_sleep() and umtx_wakeup() to no longer use
      vm_fault_page_quick().  Calling the VM fault code incurs a huge
      overhead and creates massive contention when many threads are
      using these calls.
      The new code uses fuword(), translate to the physical address via
      PTmap, and has very low overhead and basically zero contention.
    * Instead, impose a mandatory timeout for umtx_sleep() and cap it
      at 2 seconds (adjustable via sysctl kern.umtx_timeout_max, set
      in microseconds).  When the memory mapping underpinning a umtx
      changes, userland will not stall for more than 2 seconds.
    * The common remapping case caused by fork() is handled by the kernel
      by immediately waking up all sleeping umtx_sleep() calls for the
      related process.
    * Any other copy-on-write or remapping cases will stall no more
      than the maximum timeout (2 seconds).  This might include paging
      to/from swap, for example, which can remap the physical page
      underpinning the umtx.  This could also include user application
      snafus or weirdness.
    * umtx_sleep() and umtx_wakeup() still translate the user virtual
      address to a physical address for the tsleep() and wakeup() operation.
      This is done via a fault-protected access to the PTmap (the page-table

Summary of changes:
 sys/kern/kern_fork.c               |   8 ++-
 sys/kern/kern_synch.c              |  29 +++++++--
 sys/kern/kern_umtx.c               | 126 +++++++++++++++----------------------
 sys/platform/pc64/x86_64/pmap.c    |  23 +++++++
 sys/platform/pc64/x86_64/support.s |  24 +++++++
 sys/sys/systm.h                    |   2 +
 sys/vm/pmap.h                      |   1 +
 7 files changed, 130 insertions(+), 83 deletions(-)

DragonFly BSD source repository

More information about the Commits mailing list