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Paolo Bonzini authored
The following race can cause lost map update events:

         cpu1                            cpu2

                                apic_map_dirty = true
  ------------------------------------------------------------
                                kvm_recalculate_apic_map:
                                     pass check
                                         mutex_lock(&kvm->arch.apic_map_lock);
                                         if (!kvm->arch.apic_map_dirty)
                                     and in process of updating map
  -------------------------------------------------------------
    other calls to
       apic_map_dirty = true         might be too late for affected cpu
  -------------------------------------------------------------
                                     apic_map_dirty = false
  -------------------------------------------------------------
    kvm_recalculate_apic_map:
    bail out on
      if (!kvm->arch.apic_map_dirty)

To fix it, record the beginning of an update of the APIC map in
apic_map_dirty.  If another APIC map change switches apic_map_dirty
back to DIRTY during the update, kvm_recalculate_apic_map should not
make it CLEAN, and the other caller will go through the slow path.

Reported-by: default avatarIgor Mammedov <imammedo@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
44d52717
Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.