linux-kernel - Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

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Date:   Thu, 1 Sep 2016 16:49:12 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <nicholas.piggin@...il.com>
Subject: Re: [RFC][PATCH] Fix a race between rwsem and the scheduler



On 01/09/16 07:47, Benjamin Herrenschmidt wrote:
> On Wed, 2016-08-31 at 15:31 +0200, Peter Zijlstra wrote:
>> On Wed, Aug 31, 2016 at 07:28:18AM +1000, Benjamin Herrenschmidt
>> wrote:
>>
>>>
>>> On powerpc we have a sync deep in _switch to achieve that.
>>
>> OK, for giggles, could you (or Balbir) check what happens if you take
>> that sync out?
>>
>> There should be enough serialization in the generic code to cover the
>> case that code mentions.
>>
>> ARM64 has a stronger barrier in its context switch code, but that's
>> because they need to sync against external agents (like their TLB and
>> cache) and no amount of generic locking is going to cover that.
> 
> The problem is no amount of testing can tell you it works for sure :-)
> 
> I would be nervous not having a real full sync in _switch. All we have
> along the scheduler path is lwsync's and our isync based load construct
> for spin_lock, I'm not sure what other assumptions we have around that
> sync in there...
> 

I would agree, I am not sure of the assumptions either.

Balbir Singh.