netdev - Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Tue, 27 Jun 2023 18:51:16 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: Alexei Starovoitov <ast@...a.com>, Tejun Heo <tj@...nel.org>, rcu@...r.kernel.org, 
	Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kernel Team <kernel-team@...com>, Daniel Borkmann <daniel@...earbox.net>, 
	David Vernet <void@...ifault.com>, Andrii Nakryiko <andrii@...nel.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

On Tue, Jun 27, 2023 at 6:43 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> Hi,
>
> On 6/28/2023 8:56 AM, Alexei Starovoitov wrote:
> > On 6/25/23 4:15 AM, Hou Tao wrote:
> >> Hi,
> >>
> >> On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
> >>> From: Alexei Starovoitov <ast@...nel.org>
> >>>
> >>> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
> >>> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse
> >>> into
> >>> per-cpu free list the _rcu() flavor waits for RCU grace period and
> >>> then moves
> >>> objects into free_by_rcu_ttrace list where they are waiting for RCU
> >>> task trace grace period to be freed into slab.
> >> SNIP
> >>>     static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
> >>> @@ -498,8 +566,8 @@ static void free_mem_alloc_no_barrier(struct
> >>> bpf_mem_alloc *ma)
> >>>     static void free_mem_alloc(struct bpf_mem_alloc *ma)
> >>>   {
> >>> -    /* waiting_for_gp_ttrace lists was drained, but __free_rcu might
> >>> -     * still execute. Wait for it now before we freeing percpu caches.
> >>> +    /* waiting_for_gp[_ttrace] lists were drained, but RCU callbacks
> >>> +     * might still execute. Wait for them.
> >>>        *
> >>>        * rcu_barrier_tasks_trace() doesn't imply
> >>> synchronize_rcu_tasks_trace(),
> >>>        * but rcu_barrier_tasks_trace() and rcu_barrier() below are
> >>> only used
> >> I think an extra rcu_barrier() before rcu_barrier_tasks_trace() is still
> >> needed here, otherwise free_mem_alloc will not wait for inflight
> >> __free_by_rcu() and there will oops in rcu_do_batch().
> >
> > Agree. I got confused by rcu_trace_implies_rcu_gp().
> > rcu_barrier() is necessary.
> >
> > re: draining.
> > I'll switch to do if (draing) free_all; else call_rcu; scheme
> > to address potential memory leak though I wasn't able to repro it.
> For v2, it was also hard for me to reproduce the leak problem. But after
> I injected some delay by using udelay() in __free_by_rcu/__free_rcu()
> after reading c->draining, it was relatively easy to reproduce the problems.

1. Please respin htab bench.
We're still discussing patching without having the same base line.

2. 'adding udelay()' is too vague. Pls post a diff hunk of what exactly
you mean.

3. I'll send v3 shortly. Let's move discussion there.