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Date: Wed, 18 Jul 2018 12:24:38 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: Andrei Vagin <avagin@...tuozzo.com>
Cc: "Nambiar, Amritha" <amritha.nambiar@...el.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, netdev@...r.kernel.org,
davem@...emloft.net, alexander.h.duyck@...el.com,
willemdebruijn.kernel@...il.com, sridhar.samudrala@...el.com,
alexander.duyck@...il.com, edumazet@...gle.com,
hannes@...essinduktion.org, tom@...bertland.com,
tom@...ntonium.net, jasowang@...hat.com, gaowanlong@...fujitsu.com
Subject: Re: [net-next, v6, 6/7] net-sysfs: Add interface for Rx queue(s)
map per Tx queue
On Wed, 18 Jul 2018 11:22:36 -0700
Andrei Vagin <avagin@...tuozzo.com> wrote:
> On Tue, Jul 10, 2018 at 07:28:49PM -0700, Nambiar, Amritha wrote:
> > On 7/4/2018 12:20 AM, Andrei Vagin wrote:
> > > Hello Amritha,
> > >
> > > I see a following warning on 4.18.0-rc3-next-20180703.
> > > It looks like a problem is in this series.
> > >
> > > [ 1.084722] ============================================
> > > [ 1.084797] WARNING: possible recursive locking detected
> > > [ 1.084872] 4.18.0-rc3-next-20180703+ #1 Not tainted
> > > [ 1.084949] --------------------------------------------
> > > [ 1.085024] swapper/0/1 is trying to acquire lock:
> > > [ 1.085100] 00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: static_key_slow_inc+0xe/0x20
> > > [ 1.085189]
> > > [ 1.085189] but task is already holding lock:
> > > [ 1.085271] 00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: init_vqs+0x513/0x5a0
> > > [ 1.085357]
> > > [ 1.085357] other info that might help us debug this:
> > > [ 1.085450] Possible unsafe locking scenario:
> > > [ 1.085450]
> > > [ 1.085531] CPU0
> > > [ 1.085605] ----
> > > [ 1.085679] lock(cpu_hotplug_lock.rw_sem);
> > > [ 1.085753] lock(cpu_hotplug_lock.rw_sem);
> > > [ 1.085828]
> > > [ 1.085828] *** DEADLOCK ***
> > > [ 1.085828]
> > > [ 1.085916] May be due to missing lock nesting notation
> > > [ 1.085916]
> > > [ 1.085998] 3 locks held by swapper/0/1:
> > > [ 1.086074] #0: 00000000244bc7da (&dev->mutex){....}, at: __driver_attach+0x5a/0x110
> > > [ 1.086164] #1: 00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: init_vqs+0x513/0x5a0
> > > [ 1.086248] #2: 000000005cd8463f (xps_map_mutex){+.+.}, at: __netif_set_xps_queue+0x8d/0xc60
> > > [ 1.086336]
> > > [ 1.086336] stack backtrace:
> > > [ 1.086419] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.0-rc3-next-20180703+ #1
> > > [ 1.086504] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > [ 1.086587] Call Trace:
> > > [ 1.086667] dump_stack+0x85/0xcb
> > > [ 1.086744] __lock_acquire+0x68a/0x1330
> > > [ 1.086821] ? lock_acquire+0x9f/0x200
> > > [ 1.086900] ? find_held_lock+0x2d/0x90
> > > [ 1.086976] ? lock_acquire+0x9f/0x200
> > > [ 1.087051] lock_acquire+0x9f/0x200
> > > [ 1.087126] ? static_key_slow_inc+0xe/0x20
> > > [ 1.087205] cpus_read_lock+0x3e/0x80
> > > [ 1.087280] ? static_key_slow_inc+0xe/0x20
> > > [ 1.087355] static_key_slow_inc+0xe/0x20
> > > [ 1.087435] __netif_set_xps_queue+0x216/0xc60
> > > [ 1.087512] virtnet_set_affinity+0xf0/0x130
> > > [ 1.087589] init_vqs+0x51b/0x5a0
> > > [ 1.087665] virtnet_probe+0x39f/0x870
> > > [ 1.087742] virtio_dev_probe+0x170/0x220
> > > [ 1.087819] driver_probe_device+0x30b/0x480
> > > [ 1.087897] ? set_debug_rodata+0x11/0x11
> > > [ 1.087972] __driver_attach+0xe0/0x110
> > > [ 1.088064] ? driver_probe_device+0x480/0x480
> > > [ 1.088141] bus_for_each_dev+0x79/0xc0
> > > [ 1.088221] bus_add_driver+0x164/0x260
> > > [ 1.088302] ? veth_init+0x11/0x11
> > > [ 1.088379] driver_register+0x5b/0xe0
> > > [ 1.088402] ? veth_init+0x11/0x11
> > > [ 1.088402] virtio_net_driver_init+0x6d/0x90
> > > [ 1.088402] do_one_initcall+0x5d/0x34c
> > > [ 1.088402] ? set_debug_rodata+0x11/0x11
> > > [ 1.088402] ? rcu_read_lock_sched_held+0x6b/0x80
> > > [ 1.088402] kernel_init_freeable+0x1ea/0x27b
> > > [ 1.088402] ? rest_init+0xd0/0xd0
> > > [ 1.088402] kernel_init+0xa/0x110
> > > [ 1.088402] ret_from_fork+0x3a/0x50
> > > [ 1.094190] i8042: PNP: PS/2 Controller [PNP0303:KBD,PNP0f13:MOU] at 0x60,0x64 irq 1,12
> > >
> > >
> > > https://travis-ci.org/avagin/linux/jobs/399867744
> > >
> >
> > With this patch series, I introduced static_key for XPS maps
> > (xps_needed), so static_key_slow_inc() is used to switch branches. The
> > definition of static_key_slow_inc() has cpus_read_lock in place. In the
> > virtio_net driver, XPS queues are initialized after setting the
> > queue:cpu affinity in virtnet_set_affinity() which is already protected
> > within cpus_read_lock. Hence, the warning here trying to acquire
> > cpus_read_lock when it is already held.
> >
> > A quick fix for this would be to just extract netif_set_xps_queue() out
> > of the lock by simply wrapping it with another put/get_online_cpus
> > (unlock right before and hold lock right after). But this may not a
> > clean solution. It'd help if I can get suggestions on what would be a
> > clean option to fix this without extensively changing the code in
> > virtio_net. Is it mandatory to protect the affinitization with
> > read_lock? I don't see similar lock in other drivers while setting the
> > affinity.
>
> > I understand this warning should go away, but isn't it safe to
> > have multiple readers.
>
> Peter and Ingo, maybe you could explain why it isn't safe to take one
> reader lock twice?
>
> Thanks,
> Andrei
I think the issue was that some architectures, I think read lock is equivalent to a spin lock.
But maybe that is no longer true, or know one remembers.
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