Messages in this thread Patch in this message | | | From | Wanpeng Li <> | Subject | [PATCH v3] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed() | Date | Fri, 28 Aug 2015 14:55:56 +0800 |
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------------[ cut here ]------------ WARNING: CPU: 0 PID: 13 at kernel/sched/core.c:1156 do_set_cpus_allowed+0x7e/0x80() Modules linked in: CPU: 0 PID: 13 Comm: migration/0 Not tainted 4.2.0-rc1-00049-g25834c7 #2 Call Trace: dump_stack+0x4b/0x75 warn_slowpath_common+0x8b/0xc0 ? do_set_cpus_allowed+0x7e/0x80 ? do_set_cpus_allowed+0x7e/0x80 warn_slowpath_null+0x22/0x30 do_set_cpus_allowed+0x7e/0x80 cpuset_cpus_allowed_fallback+0x7c/0x170 ? cpuset_cpus_allowed+0x180/0x180 select_fallback_rq+0x221/0x280 migration_call+0xe3/0x250 notifier_call_chain+0x53/0x70 __raw_notifier_call_chain+0x1e/0x30 cpu_notify+0x28/0x50 take_cpu_down+0x22/0x40 multi_cpu_stop+0xd5/0x140 ? __stop_cpus+0x80/0x80 cpu_stopper_thread+0xbc/0x170 ? preempt_count_sub+0x9/0x50 ? _raw_spin_unlock_irq+0x37/0x50 ? _raw_spin_unlock_irqrestore+0x55/0x70 ? trace_hardirqs_on_caller+0x144/0x1e0 ? cpu_stop_should_run+0x35/0x40 ? preempt_count_sub+0x9/0x50 ? _raw_spin_unlock_irqrestore+0x41/0x70 smpboot_thread_fn+0x174/0x2f0 ? sort_range+0x30/0x30 kthread+0xc4/0xe0 ret_from_kernel_thread+0x21/0x30 ? kthread_create_on_node+0x180/0x180 ---[ end trace 15f4c86d404693b0 ]---
As Peterz pointed out:
| So the normal rules for changing task_struct::cpus_allowed are holding | both pi_lock and rq->lock, such that holding either stabilizes the mask. | | This is so that wakeup can happen without rq->lock and load-balance | without pi_lock. | | From this we already get the relaxation that we can omit acquiring | rq->lock if the task is not on the rq, because in that case | load-balancing will not apply to it. | | ** these are the rules currently tested in do_set_cpus_allowed() ** | | Now, since __set_cpus_allowed_ptr() uses task_rq_lock() which | unconditionally acquires both locks, we could get away with holding just | rq->lock when on_rq for modification because that'd still exclude | __set_cpus_allowed_ptr(), it would also work against | __kthread_bind_mask() because that assumes !on_rq. | | That said, this is all somewhat fragile. | | Now, I don't think dropping rq->lock is quite as disastrous as it | usually is because !cpu_active at this point, which means load-balance | will not interfere, but that too is somewhat fragile. | | So we end up with a choice of two fragile..
This patch fix it by following the rules for changing task_struct::cpus_allowed w/ both pi_lock and rq->lock are held.
Reported-by: kernel test robot <ying.huang@intel.com> Reported-by: Sasha Levin <sasha.levin@oracle.com> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com> --- v2 -> v3: * drop the unnecessary lockdep_unpin and unlock rq->lock v1 -> v2: * fix the silly double lock stuff * follow the rules for changing task_struct::cpus_allowed
kernel/sched/core.c | 20 ++++++++++++++++++++ 1 files changed, 20 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index b3386c6..56d19cc 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5186,6 +5186,25 @@ static void migrate_tasks(struct rq *dead_rq) BUG_ON(!next); next->sched_class->put_prev_task(rq, next); + /* + * Rules for changing task_struct::cpus_allowed are holding + * both pi_lock and rq->lock, such that holding either + * stabilizes the mask. + * + * Drop rq->lock is not quite as disastrous as it usually is + * because !cpu_active at this point, which means load-balance + * will not interfere. + */ + lockdep_unpin_lock(&rq->lock); + raw_spin_unlock(&rq->lock); + raw_spin_lock(&next->pi_lock); + raw_spin_lock(&rq->lock); + lockdep_pin_lock(&rq->lock); + if (!(task_rq(next) == rq && task_on_rq_queued(next))) { + raw_spin_unlock(&next->pi_lock); + continue; + } + /* Find suitable destination for @next, with force if needed. */ dest_cpu = select_fallback_rq(dead_rq->cpu, next); @@ -5196,6 +5215,7 @@ static void migrate_tasks(struct rq *dead_rq) rq = dead_rq; raw_spin_lock(&rq->lock); } + raw_spin_unlock(&next->pi_lock); } rq->stop = stop; -- 1.7.1
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