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

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Message-ID: <20190131020150.zbys2r6me7em2jnl@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 30 Jan 2019 18:01:52 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Wed, Jan 30, 2019 at 04:32:12PM -0500, Waiman Long wrote:
> On 01/30/2019 04:11 PM, Waiman Long wrote:
> > On 01/30/2019 03:10 PM, Alexei Starovoitov wrote:
> >> On Wed, Jan 30, 2019 at 02:42:23PM -0500, Waiman Long wrote:
> >>> 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.
> >> I see. Got it.
> >>
> >>>> 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.
> >> Yes, but looking through the code it's not clear to me that it's safe
> >> to mix non_owner() versions with regular.
> >> bpf/stackmap.c does down_read_trylock + up_read.
> >> If we add new down_read_trylock_non_owner that set the owner to
> >> NULL | RWSEM_* bits is this safe with conccurent read/write
> >> that do regular versions?
> >> rwsem_can_spin_on_owner() does:
> >>         if (owner) {
> >>                 ret = is_rwsem_owner_spinnable(owner) &&
> >>                       owner_on_cpu(owner);
> >> that looks correct.
> >> For a second I thought there could be fault here due to non_owner.
> >> But there could be other places where it's assumed that owner
> >> is never null?
> > The content of owner is not the cause of the lockdep warning. The
> > lockdep code assumes that the task that acquires the lock will release
> > it some time later. That is not the case when you need to acquire the
> > lock by one task and released by another. In this case, you have to use
> > the non_owner version of down/up_read which disable the lockdep
> > acquire/release tracking. That will be the only difference between the
> > two set of APIs.
> >
> > Cheers,
> > Longman
> 
> BTW, you may want to do something like that to make sure that the lock
> ownership is probably tracked.
> 
> -Longman
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index d43b145..79eef9d 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -338,6 +338,13 @@ static void stack_map_get_build_id_offset(struct
> bpf_stack_
>         } else {
>                 work->sem = &current->mm->mmap_sem;
>                 irq_work_queue(&work->irq_work);
> +
> +               /*
> +                * The irq_work will release the mmap_sem with
> +                * up_read_non_owner(). The rwsem_release() is called
> +                * here to release the lock from lockdep's perspective.
> +                */
> +               rwsem_release(&current->mm->mmap_sem.dep_map, 1, _RET_IP_);

are you saying the above is enough coupled with up_read_non_owner?
Looking at how amdgpu is using this api... I think they just use non_owner
version when doing things from different task.
So I don't think pairing non_owner with non_owner is strictly necessary.
It seems fine to use down_read_trylock() with above rwsem_release() hack
plus up_read_non_owner().
Am I missing something?