• Steven Rostedt's avatar
    rcu: Don't call wakeup() with rcu_node structure ->lock held · 016a8d5b
    Steven Rostedt authored
    This commit fixes a lockdep-detected deadlock by moving a wake_up()
    call out from a rnp->lock critical section.  Please see below for
    the long version of this story.
    
    On Tue, 2013-05-28 at 16:13 -0400, Dave Jones wrote:
    
    > [12572.705832] ======================================================
    > [12572.750317] [ INFO: possible circular locking dependency detected ]
    > [12572.796978] 3.10.0-rc3+ #39 Not tainted
    > [12572.833381] -------------------------------------------------------
    > [12572.862233] trinity-child17/31341 is trying to acquire lock:
    > [12572.870390]  (rcu_node_0){..-.-.}, at: [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
    > [12572.878859]
    > but task is already holding lock:
    > [12572.894894]  (&ctx->lock){-.-...}, at: [<ffffffff811390ed>] perf_lock_task_context+0x7d/0x2d0
    > [12572.903381]
    > which lock already depends on the new lock.
    >
    > [12572.927541]
    > the existing dependency chain (in reverse order) is:
    > [12572.943736]
    > -> #4 (&ctx->lock){-.-...}:
    > [12572.960032]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12572.968337]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
    > [12572.976633]        [<ffffffff8113c987>] __perf_event_task_sched_out+0x2e7/0x5e0
    > [12572.984969]        [<ffffffff81088953>] perf_event_task_sched_out+0x93/0xa0
    > [12572.993326]        [<ffffffff816ea0bf>] __schedule+0x2cf/0x9c0
    > [12573.001652]        [<ffffffff816eacfe>] schedule_user+0x2e/0x70
    > [12573.009998]        [<ffffffff816ecd64>] retint_careful+0x12/0x2e
    > [12573.018321]
    > -> #3 (&rq->lock){-.-.-.}:
    > [12573.034628]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12573.042930]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
    > [12573.051248]        [<ffffffff8108e6a7>] wake_up_new_task+0xb7/0x260
    > [12573.059579]        [<ffffffff810492f5>] do_fork+0x105/0x470
    > [12573.067880]        [<ffffffff81049686>] kernel_thread+0x26/0x30
    > [12573.076202]        [<ffffffff816cee63>] rest_init+0x23/0x140
    > [12573.084508]        [<ffffffff81ed8e1f>] start_kernel+0x3f1/0x3fe
    > [12573.092852]        [<ffffffff81ed856f>] x86_64_start_reservations+0x2a/0x2c
    > [12573.101233]        [<ffffffff81ed863d>] x86_64_start_kernel+0xcc/0xcf
    > [12573.109528]
    > -> #2 (&p->pi_lock){-.-.-.}:
    > [12573.125675]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12573.133829]        [<ffffffff816ebe9b>] _raw_spin_lock_irqsave+0x4b/0x90
    > [12573.141964]        [<ffffffff8108e881>] try_to_wake_up+0x31/0x320
    > [12573.150065]        [<ffffffff8108ebe2>] default_wake_function+0x12/0x20
    > [12573.158151]        [<ffffffff8107bbf8>] autoremove_wake_function+0x18/0x40
    > [12573.166195]        [<ffffffff81085398>] __wake_up_common+0x58/0x90
    > [12573.174215]        [<ffffffff81086909>] __wake_up+0x39/0x50
    > [12573.182146]        [<ffffffff810fc3da>] rcu_start_gp_advanced.isra.11+0x4a/0x50
    > [12573.190119]        [<ffffffff810fdb09>] rcu_start_future_gp+0x1c9/0x1f0
    > [12573.198023]        [<ffffffff810fe2c4>] rcu_nocb_kthread+0x114/0x930
    > [12573.205860]        [<ffffffff8107a91d>] kthread+0xed/0x100
    > [12573.213656]        [<ffffffff816f4b1c>] ret_from_fork+0x7c/0xb0
    > [12573.221379]
    > -> #1 (&rsp->gp_wq){..-.-.}:
    > [12573.236329]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12573.243783]        [<ffffffff816ebe9b>] _raw_spin_lock_irqsave+0x4b/0x90
    > [12573.251178]        [<ffffffff810868f3>] __wake_up+0x23/0x50
    > [12573.258505]        [<ffffffff810fc3da>] rcu_start_gp_advanced.isra.11+0x4a/0x50
    > [12573.265891]        [<ffffffff810fdb09>] rcu_start_future_gp+0x1c9/0x1f0
    > [12573.273248]        [<ffffffff810fe2c4>] rcu_nocb_kthread+0x114/0x930
    > [12573.280564]        [<ffffffff8107a91d>] kthread+0xed/0x100
    > [12573.287807]        [<ffffffff816f4b1c>] ret_from_fork+0x7c/0xb0
    
    Notice the above call chain.
    
    rcu_start_future_gp() is called with the rnp->lock held. Then it calls
    rcu_start_gp_advance, which does a wakeup.
    
    You can't do wakeups while holding the rnp->lock, as that would mean
    that you could not do a rcu_read_unlock() while holding the rq lock, or
    any lock that was taken while holding the rq lock. This is because...
    (See below).
    
    > [12573.295067]
    > -> #0 (rcu_node_0){..-.-.}:
    > [12573.309293]        [<ffffffff810b8d36>] __lock_acquire+0x1786/0x1af0
    > [12573.316568]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12573.323825]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
    > [12573.331081]        [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
    > [12573.338377]        [<ffffffff810760a6>] __rcu_read_unlock+0x96/0xa0
    > [12573.345648]        [<ffffffff811391b3>] perf_lock_task_context+0x143/0x2d0
    > [12573.352942]        [<ffffffff8113938e>] find_get_context+0x4e/0x1f0
    > [12573.360211]        [<ffffffff811403f4>] SYSC_perf_event_open+0x514/0xbd0
    > [12573.367514]        [<ffffffff81140e49>] SyS_perf_event_open+0x9/0x10
    > [12573.374816]        [<ffffffff816f4dd4>] tracesys+0xdd/0xe2
    
    Notice the above trace.
    
    perf took its own ctx->lock, which can be taken while holding the rq
    lock. While holding this lock, it did a rcu_read_unlock(). The
    perf_lock_task_context() basically looks like:
    
    rcu_read_lock();
    raw_spin_lock(ctx->lock);
    rcu_read_unlock();
    
    Now, what looks to have happened, is that we scheduled after taking that
    first rcu_read_lock() but before taking the spin lock. When we scheduled
    back in and took the ctx->lock, the following rcu_read_unlock()
    triggered the "special" code.
    
    The rcu_read_unlock_special() takes the rnp->lock, which gives us a
    possible deadlock scenario.
    
    	CPU0		CPU1		CPU2
    	----		----		----
    
    				     rcu_nocb_kthread()
        lock(rq->lock);
    		    lock(ctx->lock);
    				     lock(rnp->lock);
    
    				     wake_up();
    
    				     lock(rq->lock);
    
    		    rcu_read_unlock();
    
    		    rcu_read_unlock_special();
    
    		    lock(rnp->lock);
        lock(ctx->lock);
    
    **** DEADLOCK ****
    
    > [12573.382068]
    > other info that might help us debug this:
    >
    > [12573.403229] Chain exists of:
    >   rcu_node_0 --> &rq->lock --> &ctx->lock
    >
    > [12573.424471]  Possible unsafe locking scenario:
    >
    > [12573.438499]        CPU0                    CPU1
    > [12573.445599]        ----                    ----
    > [12573.452691]   lock(&ctx->lock);
    > [12573.459799]                                lock(&rq->lock);
    > [12573.467010]                                lock(&ctx->lock);
    > [12573.474192]   lock(rcu_node_0);
    > [12573.481262]
    >  *** DEADLOCK ***
    >
    > [12573.501931] 1 lock held by trinity-child17/31341:
    > [12573.508990]  #0:  (&ctx->lock){-.-...}, at: [<ffffffff811390ed>] perf_lock_task_context+0x7d/0x2d0
    > [12573.516475]
    > stack backtrace:
    > [12573.530395] CPU: 1 PID: 31341 Comm: trinity-child17 Not tainted 3.10.0-rc3+ #39
    > [12573.545357]  ffffffff825b4f90 ffff880219f1dbc0 ffffffff816e375b ffff880219f1dc00
    > [12573.552868]  ffffffff816dfa5d ffff880219f1dc50 ffff88023ce4d1f8 ffff88023ce4ca40
    > [12573.560353]  0000000000000001 0000000000000001 ffff88023ce4d1f8 ffff880219f1dcc0
    > [12573.567856] Call Trace:
    > [12573.575011]  [<ffffffff816e375b>] dump_stack+0x19/0x1b
    > [12573.582284]  [<ffffffff816dfa5d>] print_circular_bug+0x200/0x20f
    > [12573.589637]  [<ffffffff810b8d36>] __lock_acquire+0x1786/0x1af0
    > [12573.596982]  [<ffffffff810918f5>] ? sched_clock_cpu+0xb5/0x100
    > [12573.604344]  [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
    > [12573.611652]  [<ffffffff811054ff>] ? rcu_read_unlock_special+0x9f/0x4c0
    > [12573.619030]  [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
    > [12573.626331]  [<ffffffff811054ff>] ? rcu_read_unlock_special+0x9f/0x4c0
    > [12573.633671]  [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
    > [12573.640992]  [<ffffffff811390ed>] ? perf_lock_task_context+0x7d/0x2d0
    > [12573.648330]  [<ffffffff810b429e>] ? put_lock_stats.isra.29+0xe/0x40
    > [12573.655662]  [<ffffffff813095a0>] ? delay_tsc+0x90/0xe0
    > [12573.662964]  [<ffffffff810760a6>] __rcu_read_unlock+0x96/0xa0
    > [12573.670276]  [<ffffffff811391b3>] perf_lock_task_context+0x143/0x2d0
    > [12573.677622]  [<ffffffff81139070>] ? __perf_event_enable+0x370/0x370
    > [12573.684981]  [<ffffffff8113938e>] find_get_context+0x4e/0x1f0
    > [12573.692358]  [<ffffffff811403f4>] SYSC_perf_event_open+0x514/0xbd0
    > [12573.699753]  [<ffffffff8108cd9d>] ? get_parent_ip+0xd/0x50
    > [12573.707135]  [<ffffffff810b71fd>] ? trace_hardirqs_on_caller+0xfd/0x1c0
    > [12573.714599]  [<ffffffff81140e49>] SyS_perf_event_open+0x9/0x10
    > [12573.721996]  [<ffffffff816f4dd4>] tracesys+0xdd/0xe2
    
    This commit delays the wakeup via irq_work(), which is what
    perf and ftrace use to perform wakeups in critical sections.
    Reported-by: 's avatarDave Jones <davej@redhat.com>
    Signed-off-by: 's avatarSteven Rostedt <rostedt@goodmis.org>
    Signed-off-by: 's avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    016a8d5b
rcutree.h 22.5 KB