• Don Zickus's avatar
    x86, reboot: Use NMI instead of REBOOT_VECTOR to stop cpus · 3603a251
    Don Zickus authored
    A recent discussion started talking about the locking on the
    pstore fs and how it relates to the kmsg infrastructure.  We
    noticed it was possible for userspace to r/w to the pstore fs
    (grabbing the locks in the process) and block the panic path
    from r/w to the same fs.
    
    The reason was the cpu with the lock could be doing work while
    the crashing cpu is panic'ing.  Busting those spinlocks might
    cause those cpus to step on each other's data.  Fine, fair
    enough.
    
    It was suggested it would be nice to serialize the panic path
    (ie stop the other cpus) and have only one cpu running.  This
    would allow us to bust the spinlocks and not worry about another
    cpu stepping on the data.
    
    Of course, smp_send_stop() does this in the panic case.
    kmsg_dump() would have to be moved to be called after it.  Easy
    enough.
    
    The only problem is on x86 the smp_send_stop() function calls
    the REBOOT_VECTOR.  Any cpu with irqs disabled (which pstore and
    its backend ERST would do), block this IPI and thus do not stop.
     This makes it difficult to reliably log data to the pstore fs.
    
    The patch below switches from the REBOOT_VECTOR to NMI (and
    mimics what kdump does).  Switching to NMI allows us to deliver
    the IPI when irqs are disabled, increasing the reliability of
    this function.
    
    However, Andi carefully noted that on some machines this
    approach does not work because of broken BIOSes or whatever.
    
    To help accomodate this, the next couple of patches will run a
    selftest and provide a knob to disable.
    
    V2:
      uses atomic ops to serialize the cpu that shuts everyone down
    V3:
      comment cleanup
    Signed-off-by: default avatarDon Zickus <dzickus@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Robert Richter <robert.richter@amd.com>
    Cc: seiji.aguchi@hds.com
    Cc: vgoyal@redhat.com
    Cc: mjg@redhat.com
    Cc: tony.luck@intel.com
    Cc: gong.chen@intel.com
    Cc: satoru.moriya@hds.com
    Cc: avi@redhat.com
    Cc: Andi Kleen <andi@firstfloor.org>
    Link: http://lkml.kernel.org/r/1318533267-18880-2-git-send-email-dzickus@redhat.comSigned-off-by: default avatarIngo Molnar <mingo@elte.hu>
    3603a251
smp.c 8.08 KB