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

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Message-ID: <20190130193040.3hmh6g7efk5z3g2j@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 30 Jan 2019 11:30:41 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     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,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH bpf-next 2/4] bpf: fix lockdep false positive in stackmap

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.

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.