[PATCH] mm, memcg: remove hotplug locking from try_charge

From: Michal Hocko
Date: Wed Aug 30 2017 - 10:09:01 EST

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@xxxxxxxx>
---
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