> With RT_FULL we get the below wreckage:
The code that this applies to has itself been fully frobbed as of the
current merge window. I believe that it should now work in -rt as is,
but who knows? ;-)
Thanx, Paul
> [ 126.060484] =======================================================
> [ 126.060486] [ INFO: possible circular locking dependency detected ]
> [ 126.060489] 3.0.1-rt10+ #30
> [ 126.060490] -------------------------------------------------------
> [ 126.060492] irq/24-eth0/1235 is trying to acquire lock:
> [ 126.060495] (&(lock)->wait_lock#2){+.+...}, at: [<ffffffff81501c81>] rt_mutex_slowunlock+0x16/0x55
> [ 126.060503]
> [ 126.060504] but task is already holding lock:
> [ 126.060506] (&p->pi_lock){-...-.}, at: [<ffffffff81074fdc>] try_to_wake_up+0x35/0x429
> [ 126.060511]
> [ 126.060511] which lock already depends on the new lock.
> [ 126.060513]
> [ 126.060514]
> [ 126.060514] the existing dependency chain (in reverse order) is:
> [ 126.060516]
> [ 126.060516] -> #1 (&p->pi_lock){-...-.}:
> [ 126.060519] [<ffffffff810afe9e>] lock_acquire+0x145/0x18a
> [ 126.060524] [<ffffffff8150291e>] _raw_spin_lock_irqsave+0x4b/0x85
> [ 126.060527] [<ffffffff810b5aa4>] task_blocks_on_rt_mutex+0x36/0x20f
> [ 126.060531] [<ffffffff815019bb>] rt_mutex_slowlock+0xd1/0x15a
> [ 126.060534] [<ffffffff81501ae3>] rt_mutex_lock+0x2d/0x2f
> [ 126.060537] [<ffffffff810d9020>] rcu_boost+0xad/0xde
> [ 126.060541] [<ffffffff810d90ce>] rcu_boost_kthread+0x7d/0x9b
> [ 126.060544] [<ffffffff8109a760>] kthread+0x99/0xa1
> [ 126.060547] [<ffffffff81509b14>] kernel_thread_helper+0x4/0x10
> [ 126.060551]
> [ 126.060552] -> #0 (&(lock)->wait_lock#2){+.+...}:
> [ 126.060555] [<ffffffff810af1b8>] __lock_acquire+0x1157/0x1816
> [ 126.060558] [<ffffffff810afe9e>] lock_acquire+0x145/0x18a
> [ 126.060561] [<ffffffff8150279e>] _raw_spin_lock+0x40/0x73
> [ 126.060564] [<ffffffff81501c81>] rt_mutex_slowunlock+0x16/0x55
> [ 126.060566] [<ffffffff81501ce7>] rt_mutex_unlock+0x27/0x29
> [ 126.060569] [<ffffffff810d9f86>] rcu_read_unlock_special+0x17e/0x1c4
> [ 126.060573] [<ffffffff810da014>] __rcu_read_unlock+0x48/0x89
> [ 126.060576] [<ffffffff8106847a>] select_task_rq_rt+0xc7/0xd5
> [ 126.060580] [<ffffffff8107511c>] try_to_wake_up+0x175/0x429
> [ 126.060583] [<ffffffff81075425>] wake_up_process+0x15/0x17
> [ 126.060585] [<ffffffff81080a51>] wakeup_softirqd+0x24/0x26
> [ 126.060590] [<ffffffff81081df9>] irq_exit+0x49/0x55
> [ 126.060593] [<ffffffff8150a3bd>] smp_apic_timer_interrupt+0x8a/0x98
> [ 126.060597] [<ffffffff81509793>] apic_timer_interrupt+0x13/0x20
> [ 126.060600] [<ffffffff810d5952>] irq_forced_thread_fn+0x1b/0x44
> [ 126.060603] [<ffffffff810d582c>] irq_thread+0xde/0x1af
> [ 126.060606] [<ffffffff8109a760>] kthread+0x99/0xa1
> [ 126.060608] [<ffffffff81509b14>] kernel_thread_helper+0x4/0x10
> [ 126.060611]
> [ 126.060612] other info that might help us debug this:
> [ 126.060614]
> [ 126.060615] Possible unsafe locking scenario:
> [ 126.060616]
> [ 126.060617] CPU0 CPU1
> [ 126.060619] ---- ----
> [ 126.060620] lock(&p->pi_lock);
> [ 126.060623] lock(&(lock)->wait_lock);
> [ 126.060625] lock(&p->pi_lock);
> [ 126.060627] lock(&(lock)->wait_lock);
> [ 126.060629]
> [ 126.060629] *** DEADLOCK ***
> [ 126.060630]
> [ 126.060632] 1 lock held by irq/24-eth0/1235:
> [ 126.060633] #0: (&p->pi_lock){-...-.}, at: [<ffffffff81074fdc>] try_to_wake_up+0x35/0x429
> [ 126.060638]
> [ 126.060638] stack backtrace:
> [ 126.060641] Pid: 1235, comm: irq/24-eth0 Not tainted 3.0.1-rt10+ #30
> [ 126.060643] Call Trace:
> [ 126.060644] <IRQ> [<ffffffff810acbde>] print_circular_bug+0x289/0x29a
> [ 126.060651] [<ffffffff810af1b8>] __lock_acquire+0x1157/0x1816
> [ 126.060655] [<ffffffff810ab3aa>] ? trace_hardirqs_off_caller+0x1f/0x99
> [ 126.060658] [<ffffffff81501c81>] ? rt_mutex_slowunlock+0x16/0x55
> [ 126.060661] [<ffffffff810afe9e>] lock_acquire+0x145/0x18a
> [ 126.060664] [<ffffffff81501c81>] ? rt_mutex_slowunlock+0x16/0x55
> [ 126.060668] [<ffffffff8150279e>] _raw_spin_lock+0x40/0x73
> [ 126.060671] [<ffffffff81501c81>] ? rt_mutex_slowunlock+0x16/0x55
> [ 126.060674] [<ffffffff810d9655>] ? rcu_report_qs_rsp+0x87/0x8c
> [ 126.060677] [<ffffffff81501c81>] rt_mutex_slowunlock+0x16/0x55
> [ 126.060680] [<ffffffff810d9ea3>] ? rcu_read_unlock_special+0x9b/0x1c4
> [ 126.060683] [<ffffffff81501ce7>] rt_mutex_unlock+0x27/0x29
> [ 126.060687] [<ffffffff810d9f86>] rcu_read_unlock_special+0x17e/0x1c4
> [ 126.060690] [<ffffffff810da014>] __rcu_read_unlock+0x48/0x89
> [ 126.060693] [<ffffffff8106847a>] select_task_rq_rt+0xc7/0xd5
> [ 126.060696] [<ffffffff810683da>] ? select_task_rq_rt+0x27/0xd5
> [ 126.060701] [<ffffffff810a852a>] ? clockevents_program_event+0x8e/0x90
> [ 126.060704] [<ffffffff8107511c>] try_to_wake_up+0x175/0x429
> [ 126.060708] [<ffffffff810a95dc>] ? tick_program_event+0x1f/0x21
> [ 126.060711] [<ffffffff81075425>] wake_up_process+0x15/0x17
> [ 126.060715] [<ffffffff81080a51>] wakeup_softirqd+0x24/0x26
> [ 126.060718] [<ffffffff81081df9>] irq_exit+0x49/0x55
> [ 126.060721] [<ffffffff8150a3bd>] smp_apic_timer_interrupt+0x8a/0x98
> [ 126.060724] [<ffffffff81509793>] apic_timer_interrupt+0x13/0x20
> [ 126.060726] <EOI> [<ffffffff81072855>] ? migrate_disable+0x75/0x12d
> [ 126.060733] [<ffffffff81080a61>] ? local_bh_disable+0xe/0x1f
> [ 126.060736] [<ffffffff81080a70>] ? local_bh_disable+0x1d/0x1f
> [ 126.060739] [<ffffffff810d5952>] irq_forced_thread_fn+0x1b/0x44
> [ 126.060742] [<ffffffff81502ac0>] ? _raw_spin_unlock_irq+0x3b/0x59
> [ 126.060745] [<ffffffff810d582c>] irq_thread+0xde/0x1af
> [ 126.060748] [<ffffffff810d5937>] ? irq_thread_fn+0x3a/0x3a
> [ 126.060751] [<ffffffff810d574e>] ? irq_finalize_oneshot+0xd1/0xd1
> [ 126.060754] [<ffffffff810d574e>] ? irq_finalize_oneshot+0xd1/0xd1
> [ 126.060757] [<ffffffff8109a760>] kthread+0x99/0xa1
> [ 126.060761] [<ffffffff81509b14>] kernel_thread_helper+0x4/0x10
> [ 126.060764] [<ffffffff81069ed7>] ? finish_task_switch+0x87/0x10a
> [ 126.060768] [<ffffffff81502ec4>] ? retint_restore_args+0xe/0xe
> [ 126.060771] [<ffffffff8109a6c7>] ? __init_kthread_worker+0x8c/0x8c
> [ 126.060774] [<ffffffff81509b10>] ? gs_change+0xb/0xb
>
> Because irq_exit() does:
>
> void irq_exit(void)
> {
> account_system_vtime(current);
> trace_hardirq_exit();
> sub_preempt_count(IRQ_EXIT_OFFSET);
> if (!in_interrupt() && local_softirq_pending())
> invoke_softirq();
>
> ...
> }
>
> Which triggers a wakeup, which uses RCU, now if the interrupted task has
> t->rcu_read_unlock_special set, the rcu usage from the wakeup will end
> up in rcu_read_unlock_special(). rcu_read_unlock_special() will test
> for in_irq(), which will fail as we just decremented preempt_count
> with IRQ_EXIT_OFFSET, and in_sering_softirq(), which for
> PREEMPT_RT_FULL reads:
>
> int in_serving_softirq(void)
> {
> int res;
>
> preempt_disable();
> res = __get_cpu_var(local_softirq_runner) == current;
> preempt_enable();
> return res;
> }
>
> Which will thus also fail, resulting in the above wreckage.
>
> The 'somewhat' ugly solution is to open-code the preempt_count() test
> in rcu_read_unlock_special().
>
> Also, we're not at all sure how ->rcu_read_unlock_special gets set
> here... so this is very likely a bandaid and more thought is required.
>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 564e3927e7b0..429a2f144e19 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -524,7 +524,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> }
>
> /* Hardware IRQ handlers cannot block, complain if they get here. */
> - if (in_irq() || in_serving_softirq()) {
> + if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) {
> lockdep_rcu_suspicious(__FILE__, __LINE__,
> "rcu_read_unlock() from irq or softirq with blocking in critical section!!!\n");
> pr_alert("->rcu_read_unlock_special: %#x (b: %d, enq: %d nq: %d)\n",
