netdev - Re: [PATCH bpf-next 2/4] bpf: fix lockdep false positive in stackmap

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Message-ID: <9246a76f-e8d4-66fc-d901-374169c3e709@redhat.com>
Date:   Wed, 30 Jan 2019 14:42:23 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, edumazet@...gle.com, jannh@...gle.com,
        netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf-next 2/4] bpf: fix lockdep false positive in stackmap

On 01/30/2019 02:30 PM, Alexei Starovoitov wrote:
> On Wed, Jan 30, 2019 at 11:15:30AM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 29, 2019 at 08:04:56PM -0800, Alexei Starovoitov wrote:
>>> Lockdep warns about false positive:
>> This is not a false positive, and you probably also need to use
>> down_read_non_owner() to match this up_read_non_owner().
>>
>> {up,down}_read() and {up,down}_read_non_owner() are not only different
>> in the lockdep annotation; there is also optimistic spin stuff that
>> relies on 'owner' tracking.
> Can you point out in the code the spin bit?
> As far as I can see sem->owner is debug only feature.
> All owner checks are done under CONFIG_DEBUG_RWSEMS.

No, sem->owner is mainly for performing optimistic spinning which is a
performance feature to make rwsem writer-lock performs similar to mutex.
The debugging part is just an add-on. It is not the reason for the
presence of sem->owner.

> Also there is no down_read_trylock_non_owner() at the moment.
> We can argue about it for -next, but I'd rather silence lockdep
> with this patch today.
>
We can add down_read_trylock_non_owner() if there is a need for it. It
should be easy to do.

Cheers,
Longman

down_read_trylock_non_owner(