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Date:   Wed, 30 Aug 2017 16:15:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Artem Savkov <asavkov@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org
Subject: Re: possible circular locking dependency
 mmap_sem/cpu_hotplug_lock.rw_sem

On Wed 16-08-17 16:07:21, Thomas Gleixner wrote:
> On Mon, 7 Aug 2017, Artem Savkov wrote:
> 
> +Cc mm folks ...

Ups, this has fallen through cracks

> > Hello,
> > 
> > After commit fc8dffd "cpu/hotplug: Convert hotplug locking to percpu rwsem"
> > the following lockdep splat started showing up on some systems while running
> > ltp's madvise06 test (right after first dirty_pages call [1]).
> >
> > [1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/madvise/madvise06.c#L136
> > 
> > [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
[...]
> > [21002.993812] 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);

OK, this smells like the same thing we had to address for
drain_all_pages by a459eeb7b852 ("mm, page_alloc: do not depend on cpu
hotplug locks inside the allocator"). try_charge might be deep in the
call path so taking cpu_hotplug_lock just calls for troubles.

I have of course forgot all the subtle details about drain_all_pages but
re-reading the changelog it seems that we can get along with droping
{get,put}_online_cpus in because drain_local_stock (which is called from
the WQ context as well) is disabling irqs and _always_ operates on the
local cpu stock. So we cannot possibly race with the memory hotplug
AFAICS.

So what do you think about the following patch?
---
>From fefd7533a95050e63b22206b76259fe098e7991e Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.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.

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/memcontrol.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b9cf3cf4a3d0..6a97e827d282 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);
@@ -1807,7 +1811,6 @@ 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);
@@ -1826,7 +1829,6 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 		}
 	}
 	put_cpu();
-	put_online_cpus();
 	mutex_unlock(&percpu_charge_mutex);
 }
 
-- 
2.13.2

-- 
Michal Hocko
SUSE Labs

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