[lkml]   [2018]   [Dec]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load
On 12/03/2018 12:08 PM, Davidlohr Bueso wrote:
> On 2018-11-30 07:10, Prateek Sood wrote:
>> In a scenario where cpu_hotplug_lock percpu_rw_semaphore is already
>> acquired for read operation by P1 using percpu_down_read().
>> Now we have P1 in the path of releaseing the cpu_hotplug_lock and P2
>> is in the process of acquiring cpu_hotplug_lock.
>> P1                                               P2
>> percpu_up_read() path                      percpu_down_write() path
>>                                           rcu_sync_enter() //gp_state=GP_PASSED
>> rcu_sync_is_idle() //returns false        down_write(rw_sem)
>> __percpu_up_read()
>> [L] task = rcu_dereference(w->task) //NULL
>> smp_rmb()                                  [S] w->task = current
>>                                             smp_mb()
>>                                            [L] readers_active_check() //fails
>>                          schedule()
>> [S] __this_cpu_dec(read_count)
>> Since load of task can result in NULL. This can lead to missed wakeup
>> in rcuwait_wake_up(). Above sequence violated the following constraint
>> in rcuwait_wake_up():
>>      WAIT                WAKE
>> [S] tsk = current      [S] cond = true
>> MB (A)                        MB (B)
>> [L] cond          [L] tsk
> Hmm yeah we don't want rcu_wake_up() to get hoisted over the __this_cpu_dec(read_count). The smp_rmb() does not make sense to me here in the first place. Did you run into this scenario by code inspection or you actually it the issue?
> Thanks,
> Davidlohr

I have checked one issue where it seems that cpu hotplug code
path is not able to get cpu_hotplug_lock in write mode and there
is a reader pending for cpu hotplug path to release
percpu_rw_semaphore->rwsem to acquire cpu_hotplug_lock.
This caused a deadlock.

From code inspection also it seems to be not adhering to arm64
smp_rmb() constraint of load/load-store ordering guarantee.


Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project

 \ /
  Last update: 2018-12-03 20:36    [W:0.096 / U:2.648 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site