• Vivek Goyal's avatar
    blk-throttle: Correct the placement of smp_rmb() · 04a6b516
    Vivek Goyal authored
    o I was discussing what are the variable being updated without spin lock and
      why do we need barriers and Oleg pointed out that location of smp_rmb()
      should be between read of td->limits_changed and tg->limits_changed. This
      patch fixes it.
    
    o Following is one possible sequence of events. Say cpu0 is executing
      throtl_update_blkio_group_read_bps() and cpu1 is executing
      throtl_process_limit_change().
    
     cpu0                                                cpu1
    
     tg->limits_changed = true;
     smp_mb__before_atomic_inc();
     atomic_inc(&td->limits_changed);
    
                                         if (!atomic_read(&td->limits_changed))
                                                 return;
    
                                         if (tg->limits_changed)
                                                 do_something;
    
     If cpu0 has updated tg->limits_changed and td->limits_changed, we want to
     make sure that if update to td->limits_changed is visible on cpu1, then
     update to tg->limits_changed should also be visible.
    
     Oleg pointed out to ensure that we need to insert an smp_rmb() between
     td->limits_changed read and tg->limits_changed read.
    
    o I had erroneously put smp_rmb() before atomic_read(&td->limits_changed).
      This patch fixes it.
    Reported-by: default avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: default avatarVivek Goyal <vgoyal@redhat.com>
    Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
    04a6b516
blk-throttle.c 28.2 KB