• Steven Rostedt (Red Hat)'s avatar
    tracing: Use current_uid() for critical time tracing · f17a5194
    Steven Rostedt (Red Hat) authored
    The irqsoff tracer records the max time that interrupts are disabled.
    There are hooks in the assembly code that calls back into the tracer when
    interrupts are disabled or enabled.
    
    When they are enabled, the tracer checks if the amount of time they
    were disabled is larger than the previous recorded max interrupts off
    time. If it is, it creates a snapshot of the currently running trace
    to store where the last largest interrupts off time was held and how
    it happened.
    
    During testing, this RCU lockdep dump appeared:
    
    [ 1257.829021] ===============================
    [ 1257.829021] [ INFO: suspicious RCU usage. ]
    [ 1257.829021] 3.10.0-rc1-test+ #171 Tainted: G        W
    [ 1257.829021] -------------------------------
    [ 1257.829021] /home/rostedt/work/git/linux-trace.git/include/linux/rcupdate.h:780 rcu_read_lock() used illegally while idle!
    [ 1257.829021]
    [ 1257.829021] other info that might help us debug this:
    [ 1257.829021]
    [ 1257.829021]
    [ 1257.829021] RCU used illegally from idle CPU!
    [ 1257.829021] rcu_scheduler_active = 1, debug_locks = 0
    [ 1257.829021] RCU used illegally from extended quiescent state!
    [ 1257.829021] 2 locks held by trace-cmd/4831:
    [ 1257.829021]  #0:  (max_trace_lock){......}, at: [<ffffffff810e2b77>] stop_critical_timing+0x1a3/0x209
    [ 1257.829021]  #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff810dae5a>] __update_max_tr+0x88/0x1ee
    [ 1257.829021]
    [ 1257.829021] stack backtrace:
    [ 1257.829021] CPU: 3 PID: 4831 Comm: trace-cmd Tainted: G        W    3.10.0-rc1-test+ #171
    [ 1257.829021] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS SDBLI944.86P 05/08/2007
    [ 1257.829021]  0000000000000001 ffff880065f49da8 ffffffff8153dd2b ffff880065f49dd8
    [ 1257.829021]  ffffffff81092a00 ffff88006bd78680 ffff88007add7500 0000000000000003
    [ 1257.829021]  ffff88006bd78680 ffff880065f49e18 ffffffff810daebf ffffffff810dae5a
    [ 1257.829021] Call Trace:
    [ 1257.829021]  [<ffffffff8153dd2b>] dump_stack+0x19/0x1b
    [ 1257.829021]  [<ffffffff81092a00>] lockdep_rcu_suspicious+0x109/0x112
    [ 1257.829021]  [<ffffffff810daebf>] __update_max_tr+0xed/0x1ee
    [ 1257.829021]  [<ffffffff810dae5a>] ? __update_max_tr+0x88/0x1ee
    [ 1257.829021]  [<ffffffff811002b9>] ? user_enter+0xfd/0x107
    [ 1257.829021]  [<ffffffff810dbf85>] update_max_tr_single+0x11d/0x12d
    [ 1257.829021]  [<ffffffff811002b9>] ? user_enter+0xfd/0x107
    [ 1257.829021]  [<ffffffff810e2b15>] stop_critical_timing+0x141/0x209
    [ 1257.829021]  [<ffffffff8109569a>] ? trace_hardirqs_on+0xd/0xf
    [ 1257.829021]  [<ffffffff811002b9>] ? user_enter+0xfd/0x107
    [ 1257.829021]  [<ffffffff810e3057>] time_hardirqs_on+0x2a/0x2f
    [ 1257.829021]  [<ffffffff811002b9>] ? user_enter+0xfd/0x107
    [ 1257.829021]  [<ffffffff8109550c>] trace_hardirqs_on_caller+0x16/0x197
    [ 1257.829021]  [<ffffffff8109569a>] trace_hardirqs_on+0xd/0xf
    [ 1257.829021]  [<ffffffff811002b9>] user_enter+0xfd/0x107
    [ 1257.829021]  [<ffffffff810029b4>] do_notify_resume+0x92/0x97
    [ 1257.829021]  [<ffffffff8154bdca>] int_signal+0x12/0x17
    
    What happened was entering into the user code, the interrupts were enabled
    and a max interrupts off was recorded. The trace buffer was saved along with
    various information about the task: comm, pid, uid, priority, etc.
    
    The uid is recorded with task_uid(tsk). But this is a macro that uses rcu_read_lock()
    to retrieve the data, and this happened to happen where RCU is blind (user_enter).
    
    As only the preempt and irqs off tracers can have this happen, and they both
    only have the tsk == current, if tsk == current, use current_uid() instead of
    task_uid(), as current_uid() does not use RCU as only current can change its uid.
    
    This fixes the RCU suspicious splat.
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    f17a5194
trace.c 149 KB