• Paul E. McKenney's avatar
    rcu: Eliminate deadlock between CPU hotplug and expedited grace periods · dd56af42
    Paul E. McKenney authored
    Currently, the expedited grace-period primitives do get_online_cpus().
    This greatly simplifies their implementation, but means that calls
    to them holding locks that are acquired by CPU-hotplug notifiers (to
    say nothing of calls to these primitives from CPU-hotplug notifiers)
    can deadlock.  But this is starting to become inconvenient, as can be
    seen here: https://lkml.org/lkml/2014/8/5/754.  The problem in this
    case is that some developers need to acquire a mutex from a CPU-hotplug
    notifier, but also need to hold it across a synchronize_rcu_expedited().
    As noted above, this currently results in deadlock.
    
    This commit avoids the deadlock and retains the simplicity by creating
    a try_get_online_cpus(), which returns false if the get_online_cpus()
    reference count could not immediately be incremented.  If a call to
    try_get_online_cpus() returns true, the expedited primitives operate as
    before.  If a call returns false, the expedited primitives fall back to
    normal grace-period operations.  This falling back of course results in
    increased grace-period latency, but only during times when CPU hotplug
    operations are actually in flight.  The effect should therefore be
    negligible during normal operation.
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
    Tested-by: default avatarLan Tianyu <tianyu.lan@intel.com>
    dd56af42
tree.c 117 KB