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SubjectRe: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw_sem
Hi Michal,

On Wed, Aug 30, 2017 at 05:43:15PM +0200, Michal Hocko wrote:
> The previous patch is insufficient. drain_all_stock can still race with
> the memory offline callback and the underlying memcg disappear. So we
> need to be more careful and pin the css on the memcg. This patch
> instead...

Tried this on top of rc7 and it does fix the splat for me.

> ---
> From 70a5acf9bbe76d183e81a1a6b57dd5b9edc677c6 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Wed, 30 Aug 2017 16:09:01 +0200
> Subject: [PATCH] mm, memcg: remove hotplug locking from try_charge
>
> The following lockde splat has been noticed during LTP testing
>
> [21002.630252] ======================================================
> [21002.637148] WARNING: possible circular locking dependency detected
> [21002.644045] 4.13.0-rc3-next-20170807 #12 Not tainted
> [21002.649583] ------------------------------------------------------
> [21002.656492] a.out/4771 is trying to acquire lock:
> [21002.661742] (cpu_hotplug_lock.rw_sem){++++++}, at: [<ffffffff812b4668>] drain_all_stock.part.35+0x18/0x140
> [21002.672629]
> [21002.672629] but task is already holding lock:
> [21002.679137] (&mm->mmap_sem){++++++}, at: [<ffffffff8106eb35>] __do_page_fault+0x175/0x530
> [21002.688371]
> [21002.688371] which lock already depends on the new lock.
> [21002.688371]
> [21002.697505]
> [21002.697505] the existing dependency chain (in reverse order) is:
> [21002.705856]
> [21002.705856] -> #3 (&mm->mmap_sem){++++++}:
> [21002.712080] lock_acquire+0xc9/0x230
> [21002.716661] __might_fault+0x70/0xa0
> [21002.721241] _copy_to_user+0x23/0x70
> [21002.725814] filldir+0xa7/0x110
> [21002.729988] xfs_dir2_sf_getdents.isra.10+0x20c/0x2c0 [xfs]
> [21002.736840] xfs_readdir+0x1fa/0x2c0 [xfs]
> [21002.742042] xfs_file_readdir+0x30/0x40 [xfs]
> [21002.747485] iterate_dir+0x17a/0x1a0
> [21002.752057] SyS_getdents+0xb0/0x160
> [21002.756638] entry_SYSCALL_64_fastpath+0x1f/0xbe
> [21002.762371]
> [21002.762371] -> #2 (&type->i_mutex_dir_key#3){++++++}:
> [21002.769661] lock_acquire+0xc9/0x230
> [21002.774239] down_read+0x51/0xb0
> [21002.778429] lookup_slow+0xde/0x210
> [21002.782903] walk_component+0x160/0x250
> [21002.787765] link_path_walk+0x1a6/0x610
> [21002.792625] path_openat+0xe4/0xd50
> [21002.797100] do_filp_open+0x91/0x100
> [21002.801673] file_open_name+0xf5/0x130
> [21002.806429] filp_open+0x33/0x50
> [21002.810620] kernel_read_file_from_path+0x39/0x80
> [21002.816459] _request_firmware+0x39f/0x880
> [21002.821610] request_firmware_direct+0x37/0x50
> [21002.827151] request_microcode_fw+0x64/0xe0
> [21002.832401] reload_store+0xf7/0x180
> [21002.836974] dev_attr_store+0x18/0x30
> [21002.841641] sysfs_kf_write+0x44/0x60
> [21002.846318] kernfs_fop_write+0x113/0x1a0
> [21002.851374] __vfs_write+0x37/0x170
> [21002.855849] vfs_write+0xc7/0x1c0
> [21002.860128] SyS_write+0x58/0xc0
> [21002.864313] do_syscall_64+0x6c/0x1f0
> [21002.868973] return_from_SYSCALL_64+0x0/0x7a
> [21002.874317]
> [21002.874317] -> #1 (microcode_mutex){+.+.+.}:
> [21002.880748] lock_acquire+0xc9/0x230
> [21002.885322] __mutex_lock+0x88/0x960
> [21002.889894] mutex_lock_nested+0x1b/0x20
> [21002.894854] microcode_init+0xbb/0x208
> [21002.899617] do_one_initcall+0x51/0x1a9
> [21002.904481] kernel_init_freeable+0x208/0x2a7
> [21002.909922] kernel_init+0xe/0x104
> [21002.914298] ret_from_fork+0x2a/0x40
> [21002.918867]
> [21002.918867] -> #0 (cpu_hotplug_lock.rw_sem){++++++}:
> [21002.926058] __lock_acquire+0x153c/0x1550
> [21002.931112] lock_acquire+0xc9/0x230
> [21002.935688] cpus_read_lock+0x4b/0x90
> [21002.940353] drain_all_stock.part.35+0x18/0x140
> [21002.945987] try_charge+0x3ab/0x6e0
> [21002.950460] mem_cgroup_try_charge+0x7f/0x2c0
> [21002.955902] shmem_getpage_gfp+0x25f/0x1050
> [21002.961149] shmem_fault+0x96/0x200
> [21002.965621] __do_fault+0x1e/0xa0
> [21002.969905] __handle_mm_fault+0x9c3/0xe00
> [21002.975056] handle_mm_fault+0x16e/0x380
> [21002.980013] __do_page_fault+0x24a/0x530
> [21002.984968] do_page_fault+0x30/0x80
> [21002.989537] page_fault+0x28/0x30
> [21002.993812]
> [21002.993812] other info that might help us debug this:
> [21002.993812]
> [21003.002744] Chain exists of:
> [21003.002744] cpu_hotplug_lock.rw_sem --> &type->i_mutex_dir_key#3 --> &mm->mmap_sem
> [21003.002744]
> [21003.016238] Possible unsafe locking scenario:
> [21003.016238]
> [21003.022843] CPU0 CPU1
> [21003.027896] ---- ----
> [21003.032948] lock(&mm->mmap_sem);
> [21003.036741] lock(&type->i_mutex_dir_key#3);
> [21003.044419] lock(&mm->mmap_sem);
> [21003.051025] lock(cpu_hotplug_lock.rw_sem);
> [21003.055788]
> [21003.055788] *** DEADLOCK ***
> [21003.055788]
> [21003.062393] 2 locks held by a.out/4771:
> [21003.066675] #0: (&mm->mmap_sem){++++++}, at: [<ffffffff8106eb35>] __do_page_fault+0x175/0x530
> [21003.076391] #1: (percpu_charge_mutex){+.+...}, at: [<ffffffff812b4c97>] try_charge+0x397/0x6e0
>
> The problem is very similar to the one fixed by a459eeb7b852 ("mm,
> page_alloc: do not depend on cpu hotplug locks inside the allocator").
> We are taking hotplug locks while we can be sitting on top of basically
> arbitrary locks. This just calls for problems.
>
> We can get rid of {get,put}_online_cpus, fortunately. We do not have to
> be worried about races with memory hotplug because drain_local_stock,
> which is called from both the WQ draining and the memory hotplug
> contexts, is always operating on the local cpu stock with IRQs disabled.
>
> The only thing to be careful about is that the target memcg doesn't
> vanish while we are still in drain_all_stock so take a reference on
> it.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> mm/memcontrol.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b9cf3cf4a3d0..5c70f47abb3d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1765,6 +1765,10 @@ static void drain_local_stock(struct work_struct *dummy)
> struct memcg_stock_pcp *stock;
> unsigned long flags;
>
> + /*
> + * The only protection from memory hotplug vs. drain_stock races is
> + * that we always operate on local CPU stock here with IRQ disabled
> + */
> local_irq_save(flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> @@ -1807,26 +1811,27 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> if (!mutex_trylock(&percpu_charge_mutex))
> return;
> /* Notify other cpus that system-wide "drain" is running */
> - get_online_cpus();
> curcpu = get_cpu();
> for_each_online_cpu(cpu) {
> struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> struct mem_cgroup *memcg;
>
> memcg = stock->cached;
> - if (!memcg || !stock->nr_pages)
> + if (!memcg || !stock->nr_pages || !css_tryget(&memcg->css))
> continue;
> - if (!mem_cgroup_is_descendant(memcg, root_memcg))
> + if (!mem_cgroup_is_descendant(memcg, root_memcg)) {
> + css_put(&memcg->css);
> continue;
> + }
> if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> if (cpu == curcpu)
> drain_local_stock(&stock->work);
> else
> schedule_work_on(cpu, &stock->work);
> }
> + css_put(&memcg->css);
> }
> put_cpu();
> - put_online_cpus();
> mutex_unlock(&percpu_charge_mutex);
> }
>
> --
> 2.13.2
>
> --
> Michal Hocko
> SUSE Labs

--
Regards,
Artem

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