| Subject: rcu: Frob softirq test |
| From: Peter Zijlstra <a.p.zijlstra@chello.nl> |
| Date: Sat Aug 13 00:23:17 CEST 2011 |
| |
| With RT_FULL we get the below wreckage: |
| |
| [ 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@linux.vnet.ibm.com> |
| Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> |
| --- |
| kernel/rcutree_plugin.h | 2 +- |
| 1 file changed, 1 insertion(+), 1 deletion(-) |
| |
| Index: linux-stable/kernel/rcutree_plugin.h |
| =================================================================== |
| --- linux-stable.orig/kernel/rcutree_plugin.h |
| +++ linux-stable/kernel/rcutree_plugin.h |
| @@ -331,7 +331,7 @@ void rcu_read_unlock_special(struct task |
| } |
| |
| /* Hardware IRQ handlers cannot block. */ |
| - if (in_irq() || in_serving_softirq()) { |
| + if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) { |
| local_irq_restore(flags); |
| return; |
| } |
