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Date: Mon, 13 Aug 2018 14:02:43 +0200
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Tariq Toukan <tariqt@...lanox.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
Eran Ben Elisha <eranbe@...lanox.com>,
Neil Brown <neilb@...e.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, brouer@...hat.com
Subject: Re: [PATCH bpf-next V3] net/xdp: Fix suspicious RCU usage warning
On Mon, 13 Aug 2018 13:57:04 +0300
Tariq Toukan <tariqt@...lanox.com> wrote:
> On 13/08/2018 1:31 PM, Jesper Dangaard Brouer wrote:
> > On Mon, 13 Aug 2018 12:21:58 +0300
> > Tariq Toukan <tariqt@...lanox.com> wrote:
> >
> >> Fix the warning below by calling rhashtable_lookup_fast.
> >> Also, make some code movements for better quality and human
> >> readability.
> >>
> >> [ 342.450870] WARNING: suspicious RCU usage
> >> [ 342.455856] 4.18.0-rc2+ #17 Tainted: G O
> >> [ 342.462210] -----------------------------
> >> [ 342.467202] ./include/linux/rhashtable.h:481 suspicious rcu_dereference_check() usage!
> >> [ 342.476568]
> >> [ 342.476568] other info that might help us debug this:
> >> [ 342.476568]
> >> [ 342.486978]
> >> [ 342.486978] rcu_scheduler_active = 2, debug_locks = 1
> >> [ 342.495211] 4 locks held by modprobe/3934:
> >> [ 342.500265] #0: 00000000e23116b2 (mlx5_intf_mutex){+.+.}, at:
> >> mlx5_unregister_interface+0x18/0x90 [mlx5_core]
> >> [ 342.511953] #1: 00000000ca16db96 (rtnl_mutex){+.+.}, at: unregister_netdev+0xe/0x20
> >> [ 342.521109] #2: 00000000a46e2c4b (&priv->state_lock){+.+.}, at: mlx5e_close+0x29/0x60
> >> [mlx5_core]
> >> [ 342.531642] #3: 0000000060c5bde3 (mem_id_lock){+.+.}, at: xdp_rxq_info_unreg+0x93/0x6b0
> >> [ 342.541206]
> >> [ 342.541206] stack backtrace:
> >> [ 342.547075] CPU: 12 PID: 3934 Comm: modprobe Tainted: G O 4.18.0-rc2+ #17
> >> [ 342.556621] Hardware name: Dell Inc. PowerEdge R730/0H21J3, BIOS 1.5.4 10/002/2015
> >> [ 342.565606] Call Trace:
> >> [ 342.568861] dump_stack+0x78/0xb3
> >> [ 342.573086] xdp_rxq_info_unreg+0x3f5/0x6b0
> >> [ 342.578285] ? __call_rcu+0x220/0x300
> >> [ 342.582911] mlx5e_free_rq+0x38/0xc0 [mlx5_core]
> >> [ 342.588602] mlx5e_close_channel+0x20/0x120 [mlx5_core]
> >> [ 342.594976] mlx5e_close_channels+0x26/0x40 [mlx5_core]
> >> [ 342.601345] mlx5e_close_locked+0x44/0x50 [mlx5_core]
> >> [ 342.607519] mlx5e_close+0x42/0x60 [mlx5_core]
> >> [ 342.613005] __dev_close_many+0xb1/0x120
> >> [ 342.617911] dev_close_many+0xa2/0x170
> >> [ 342.622622] rollback_registered_many+0x148/0x460
> >> [ 342.628401] ? __lock_acquire+0x48d/0x11b0
> >> [ 342.633498] ? unregister_netdev+0xe/0x20
> >> [ 342.638495] rollback_registered+0x56/0x90
> >> [ 342.643588] unregister_netdevice_queue+0x7e/0x100
> >> [ 342.649461] unregister_netdev+0x18/0x20
> >> [ 342.654362] mlx5e_remove+0x2a/0x50 [mlx5_core]
> >> [ 342.659944] mlx5_remove_device+0xe5/0x110 [mlx5_core]
> >> [ 342.666208] mlx5_unregister_interface+0x39/0x90 [mlx5_core]
> >> [ 342.673038] cleanup+0x5/0xbfc [mlx5_core]
> >> [ 342.678094] __x64_sys_delete_module+0x16b/0x240
> >> [ 342.683725] ? do_syscall_64+0x1c/0x210
> >> [ 342.688476] do_syscall_64+0x5a/0x210
> >> [ 342.693025] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >>
> >> Fixes: 8d5d88527587 ("xdp: rhashtable with allocator ID to pointer mapping")
> >> Signed-off-by: Tariq Toukan <tariqt@...lanox.com>
> >> Suggested-by: Daniel Borkmann <daniel@...earbox.net>
> >> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
> >> ---
> >> net/core/xdp.c | 13 +++----------
> >> 1 file changed, 3 insertions(+), 10 deletions(-)
> >>
> >> V2 -> V3:
> >> * Fix return value test for rhashtable_remove_fast, per Jesper's comment.
> >>
> >> V1 -> V2:
> >> * Use rhashtable_lookup_fast and make some code movements, per Daniel's
> >> and Alexei's comments.
> >>
> >> Please queue to -stable v4.18.
> >>
> >> diff --git a/net/core/xdp.c b/net/core/xdp.c
> >> index 3dd99e1c04f5..8b1c7b699982 100644
> >> --- a/net/core/xdp.c
> >> +++ b/net/core/xdp.c
> >> @@ -105,16 +105,9 @@ static void __xdp_rxq_info_unreg_mem_model(struct xdp_rxq_info *xdp_rxq)
> >>
> >> mutex_lock(&mem_id_lock);
> >>
> >> - xa = rhashtable_lookup(mem_id_ht, &id, mem_id_rht_params);
> >> - if (!xa) {
> >> - mutex_unlock(&mem_id_lock);
> >> - return;
> >> - }
> >> -
> >> - err = rhashtable_remove_fast(mem_id_ht, &xa->node, mem_id_rht_params);
> >> - WARN_ON(err);
> >> -
> >> - call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
> >> + xa = rhashtable_lookup_fast(mem_id_ht, &id, mem_id_rht_params);
> >> + if (xa && !rhashtable_remove_fast(mem_id_ht, &xa->node, mem_id_rht_params))
> >> + call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
> >>
> >> mutex_unlock(&mem_id_lock);
> >> }
> >
> > I would personally prefer to write it as in the example as "== 0", look
> > at example in [1] section "Object removal", but is it semantically the
> > same to write !rhashtable_remove_fast(). So, I'm fine with this.
> >
>
> I thought the coding convention is to not explicitly compare to zero,
> just like we do not compare to NULL on page allocation, but do:
> if (!page)
If the return value is a pointer, then I use the (!ptr) check, and also
if the return value is a bool. In this case where the success is 0, I
find it slightly confusing to read if(!remove) then success-case.
> But I don't mind changing this one.
I also don't care much... if you do respin, it would be nice to do.
> > In the example[1], the sequence is wrapped in rcu_read_lock/unlock,
> > while you have not done so. The rhashtable_lookup_fast and
> > rhashtable_remove_fast calls have their own rcu_read_lock/unlock,
> > but the outer rcu_read_lock/unlock, makes sure that a RCU period
> > cannot slip in between the two calls.
> >
> > I still think your fix is correct, due to the mutex_lock. Given the
> > mutex sync removal and insert in this rhashtable.
> >
>
> Right, we rely here on the mutex to avoid the scenario you described.
> So the outer rcu lock calls are not necessary.
>
> > I do wonder if it would be better to add the outer
> > rcu_read_lock/unlock, calls if someone else reads and copy-paste
> > this code (and don't have an mutex sync scheme) ?
> >
>
> Yeah it'll be safer for future unaware developers, but I think
> reviewers should always comment and make it clear that the best
> generic reference is [1], not any specific/optimized use case.
>
> If you guys still want to me to fix this then please let me know and
> I'll re-spin.
I'll let Daniel make the choice.
> > If you think this is all fine, and want to proceed as is then you
> > have my ack:
> >
> > Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>
> >
> >
> > [1] https://lwn.net/Articles/751374/
> >
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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