linux-kernel - Re: [PATCH] locking/osq_lock: fix a data race in osq_wait

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Message-Id: <E722E6E0-26CB-440F-98D7-D182B57D1F43@lca.pw>
Date:   Mon, 27 Jan 2020 22:11:10 -0500
From:   Qian Cai <cai@....pw>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next



> On Jan 23, 2020, at 4:39 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Jan 22, 2020 at 06:54:43PM -0500, Qian Cai wrote:
>> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
>> index 1f7734949ac8..832e87966dcf 100644
>> --- a/kernel/locking/osq_lock.c
>> +++ b/kernel/locking/osq_lock.c
>> @@ -75,7 +75,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>>                 * wait for either @lock to point to us, through its Step-B, or
>>                 * wait for a new @node->next from its Step-C.
>>                 */
>> -               if (node->next) {
>> +               if (READ_ONCE(node->next)) {
>>                        next = xchg(&node->next, NULL);
>>                        if (next)
>>                                break;
> 
> This could possibly trigger the warning, but is a false positive. The
> above doesn't fix anything in that even if that load is shattered the
> code will function correctly -- it checks for any !0 value, any byte
> composite that is !0 is sufficient.
> 
> This is in fact something KCSAN compiler infrastructure could deduce.


Marco, any thought on improving KCSAN for this to reduce the false
positives?