• Steven Rostedt (VMware)'s avatar
    cgroup/tracing: Move taking of spin lock out of trace event handlers · e4f8d81c
    Steven Rostedt (VMware) authored
    It is unwise to take spin locks from the handlers of trace events.
    Mainly, because they can introduce lockups, because it introduces locks
    in places that are normally not tested. Worse yet, because trace events
    are tucked away in the include/trace/events/ directory, locks that are
    taken there are forgotten about.
    
    As a general rule, I tell people never to take any locks in a trace
    event handler.
    
    Several cgroup trace event handlers call cgroup_path() which eventually
    takes the kernfs_rename_lock spinlock. This injects the spinlock in the
    code without people realizing it. It also can cause issues for the
    PREEMPT_RT patch, as the spinlock becomes a mutex, and the trace event
    handlers are called with preemption disabled.
    
    By moving the calculation of the cgroup_path() out of the trace event
    handlers and into a macro (surrounded by a
    trace_cgroup_##type##_enabled()), then we could place the cgroup_path
    into a string, and pass that to the trace event. Not only does this
    remove the taking of the spinlock out of the trace event handler, but
    it also means that the cgroup_path() only needs to be called once (it
    is currently called twice, once to get the length to reserver the
    buffer for, and once again to get the path itself. Now it only needs to
    be done once.
    Reported-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    e4f8d81c
cgroup.h 3.18 KB