• Frederic Weisbecker's avatar
    tracing, perf_events: Protect the buffer from recursion in perf · 444a2a3b
    Frederic Weisbecker authored
    
    
    While tracing using events with perf, if one enables the
    lockdep:lock_acquire event, it will infect every other perf
    trace events.
    
    Basically, you can enable whatever set of trace events through
    perf but if this event is part of the set, the only result we
    can get is a long list of lock_acquire events of rcu read lock,
    and only that.
    
    This is because of a recursion inside perf.
    
    1) When a trace event is triggered, it will fill a per cpu
       buffer and submit it to perf.
    
    2) Perf will commit this event but will also protect some data
       using rcu_read_lock
    
    3) A recursion appears: rcu_read_lock triggers a lock_acquire
       event that will fill the per cpu event and then submit the
       buffer to perf.
    
    4) Perf detects a recursion and ignores it
    
    5) Perf continues its work on the previous event, but its buffer
       has been overwritten by the lock_acquire event, it has then
       been turned into a lock_acquire event of rcu read lock
    
    Such scenario also happens with lock_release with
    rcu_read_unlock().
    
    We could turn the rcu_read_lock() into __rcu_read_lock() to drop
    the lock debugging from perf fast path, but that would make us
    lose the rcu debugging and that doesn't prevent from other
    possible kind of recursion from perf in the future.
    
    This patch adds a recursion protection based on a counter on the
    perf trace per cpu buffers to solve the problem.
    
    -v2: Fixed lost whitespace, added reviewed-by tag
    Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
    Reviewed-by: default avatarMasami Hiramatsu <mhiramat@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Jason Baron <jbaron@redhat.com>
    LKML-Reference: <1257477185-7838-1-git-send-email-fweisbec@gmail.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    444a2a3b
ftrace_event.h 5.8 KB