• Greg Thelen's avatar
    writeback: safer lock nesting · 18484eb9
    Greg Thelen authored
    commit 2e898e4c0a3897ccd434adac5abb8330194f527b upstream.
    lock_page_memcg()/unlock_page_memcg() use spin_lock_irqsave/restore() if
    the page's memcg is undergoing move accounting, which occurs when a
    process leaves its memcg for a new one that has
    memory.move_charge_at_immigrate set.
    unlocked_inode_to_wb_begin,end() use spin_lock_irq/spin_unlock_irq() if
    the given inode is switching writeback domains.  Switches occur when
    enough writes are issued from a new domain.
    This existing pattern is thus suspicious:
        unlocked_inode_to_wb_begin(inode, &locked);
        unlocked_inode_to_wb_end(inode, locked);
    If both inode switch and process memcg migration are both in-flight then
    unlocked_inode_to_wb_end() will unconditionally enable interrupts while
    still holding the lock_page_memcg() irq spinlock.  This suggests the
    possibility of deadlock if an interrupt occurs before unlock_page_memcg().
        <interrupts mistakenly enabled>
    Due to configuration limitations this deadlock is not currently possible
    because we don't mix cgroup writeback (a cgroupv2 feature) and
    memory.move_charge_at_immigrate (a cgroupv1 feature).
    If the kernel is hacked to always claim inode switching and memcg
    moving_account, then this script triggers lockup in less than a minute:
      cd /mnt/cgroup/memory
      mkdir a b
      echo 1 > a/memory.move_charge_at_immigrate
      echo 1 > b/memory.move_charge_at_immigrate
        echo $BASHPID > a/cgroup.procs
        while true; do
          dd if=/dev/zero of=/mnt/big bs=1M count=256
      ) &
      while true; do
      done &
      sleep 1h &
      while true; do
        echo $SLEEP > a/cgroup.procs
        echo $SLEEP > b/cgroup.procs
    The deadlock does not seem possible, so it's debatable if there's any
    reason to modify the kernel.  I suggest we should to prevent future
    surprises.  And Wang Long said "this deadlock occurs three times in our
    environment", so there's more reason to apply this, even to stable.
    Stable 4.4 has minor conflicts applying this patch.  For a clean 4.4 patch
    see "[PATCH for-4.4] writeback: safer lock nesting"
    Wang Long said "this deadlock occurs three times in our environment"
    [gthelen@google.com: v4]
      Link: http://lkml.kernel.org/r/20180411084653.254724-1-gthelen@google.com
    [akpm@linux-foundation.org: comment tweaks, struct initialization simplification]
    Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613
    Link: http://lkml.kernel.org/r/20180410005908.167976-1-gthelen@google.com
    Fixes: 682aa8e1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
    Signed-off-by: default avatarGreg Thelen <gthelen@google.com>
    Reported-by: default avatarWang Long <wanglong19@meituan.com>
    Acked-by: default avatarWang Long <wanglong19@meituan.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: <stable@vger.kernel.org>	[v4.2+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    [natechancellor: Adjust context due to lack of b93b016313b3b]
    Signed-off-by: default avatarNathan Chancellor <natechancellor@gmail.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>