• Peter Zijlstra's avatar
    perf: Fix a race between ring_buffer_detach() and ring_buffer_attach() · b69cf536
    Peter Zijlstra authored
    Alexander noticed that we use RCU iteration on rb->event_list but do
    not use list_{add,del}_rcu() to add,remove entries to that list, nor
    do we observe proper grace periods when re-using the entries.
    
    Merge ring_buffer_detach() into ring_buffer_attach() such that
    attaching to the NULL buffer is detaching.
    
    Furthermore, ensure that between any 'detach' and 'attach' of the same
    event we observe the required grace period, but only when strictly
    required. In effect this means that only ioctl(.request =
    PERF_EVENT_IOC_SET_OUTPUT) will wait for a grace period, while the
    normal initial attach and final detach will not be delayed.
    
    This patch should, I think, do the right thing under all
    circumstances, the 'normal' cases all should never see the extra grace
    period, but the two cases:
    
     1) PERF_EVENT_IOC_SET_OUTPUT on an event which already has a
        ring_buffer set, will now observe the required grace period between
        removing itself from the old and attaching itself to the new buffer.
    
        This case is 'simple' in that both buffers are present in
        perf_event_set_output() one could think an unconditional
        synchronize_rcu() would be sufficient; however...
    
     2) an event that has a buffer attached, the buffer is destroyed
        (munmap) and then the event is attached to a new/different buffer
        using PERF_EVENT_IOC_SET_OUTPUT.
    
        This case is more complex because the buffer destruction does:
          ring_buffer_attach(.rb = NULL)
        followed by the ioctl() doing:
          ring_buffer_attach(.rb = foo);
    
        and we still need to observe the grace period between these two
        calls due to us reusing the event->rb_entry list_head.
    
    In order to make 2 happen we use Paul's latest cond_synchronize_rcu()
    call.
    
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Mike Galbraith <efault@gmx.de>
    Reported-by: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
    Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/20140507123526.GD13658@twins.programming.kicks-ass.netSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    b69cf536
core.c 187 KB