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SubjectRe: [PATCH] mm/memory-hotplug: Switch locking to a percpu rwsem
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On 06/30/2017 01:15 PM, Thomas Gleixner wrote:
> On Fri, 30 Jun 2017, Michal Hocko wrote:
>> So I like this simplification a lot! Even if we can get rid of the
>> stop_machine eventually this patch would be an improvement. A short
>> comment on why the per-cpu semaphore over the regular one is better
>> would be nice.
>
> Yes, will add one.
>
> The main point is that the current locking construct is evading lockdep due
> to the ability to support recursive locking, which I did not observe so
> far.
>

Like this?


[ 131.022567] ============================================
[ 131.023034] WARNING: possible recursive locking detected
[ 131.023034] 4.12.0-rc7-next-20170630 #10 Not tainted
[ 131.023034] --------------------------------------------
[ 131.023034] bash/2266 is trying to acquire lock:
[ 131.023034] (cpu_hotplug_lock.rw_sem){++++++}, at: [<ffffffff8117fcd2>] lru_add_drain_all+0x42/0x190
[ 131.023034]
but task is already holding lock:
[ 131.023034] (cpu_hotplug_lock.rw_sem){++++++}, at: [<ffffffff811d5489>] mem_hotplug_begin+0x9/0x20
[ 131.023034]
other info that might help us debug this:
[ 131.023034] Possible unsafe locking scenario:

[ 131.023034] CPU0
[ 131.023034] ----
[ 131.023034] lock(cpu_hotplug_lock.rw_sem);
[ 131.023034] lock(cpu_hotplug_lock.rw_sem);
[ 131.023034]
*** DEADLOCK ***

[ 131.023034] May be due to missing lock nesting notation

[ 131.023034] 8 locks held by bash/2266:
[ 131.023034] #0: (sb_writers#8){.+.+.+}, at: [<ffffffff811e81f8>] vfs_write+0x1a8/0x1d0
[ 131.023034] #1: (&of->mutex){+.+.+.}, at: [<ffffffff81274b2c>] kernfs_fop_write+0xfc/0x1b0
[ 131.023034] #2: (s_active#48){.+.+.+}, at: [<ffffffff81274b34>] kernfs_fop_write+0x104/0x1b0
[ 131.023034] #3: (device_hotplug_lock){+.+.+.}, at: [<ffffffff816d4810>] lock_device_hotplug_sysfs+0x10/0x40
[ 131.023034] #4: (cpu_hotplug_lock.rw_sem){++++++}, at: [<ffffffff811d5489>] mem_hotplug_begin+0x9/0x20
[ 131.023034] #5: (mem_hotplug_lock.rw_sem){++++++}, at: [<ffffffff810ada81>] percpu_down_write+0x21/0x110
[ 131.023034] #6: (&dev->mutex){......}, at: [<ffffffff816d5bd5>] device_offline+0x45/0xb0
[ 131.023034] #7: (lock#3){+.+...}, at: [<ffffffff8117fccd>] lru_add_drain_all+0x3d/0x190
[ 131.023034]
stack backtrace:
[ 131.023034] CPU: 0 PID: 2266 Comm: bash Not tainted 4.12.0-rc7-next-20170630 #10
[ 131.023034] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
[ 131.023034] Call Trace:
[ 131.023034] dump_stack+0x85/0xc7
[ 131.023034] __lock_acquire+0x1747/0x17a0
[ 131.023034] ? lru_add_drain_all+0x3d/0x190
[ 131.023034] ? __mutex_lock+0x218/0x940
[ 131.023034] ? trace_hardirqs_on+0xd/0x10
[ 131.023034] lock_acquire+0x103/0x200
[ 131.023034] ? lock_acquire+0x103/0x200
[ 131.023034] ? lru_add_drain_all+0x42/0x190
[ 131.023034] cpus_read_lock+0x3d/0x80
[ 131.023034] ? lru_add_drain_all+0x42/0x190
[ 131.023034] lru_add_drain_all+0x42/0x190
[ 131.023034] __offline_pages.constprop.25+0x5de/0x870
[ 131.023034] offline_pages+0xc/0x10
[ 131.023034] memory_subsys_offline+0x43/0x70
[ 131.023034] device_offline+0x83/0xb0
[ 131.023034] store_mem_state+0xdb/0xe0
[ 131.023034] dev_attr_store+0x13/0x20
[ 131.023034] sysfs_kf_write+0x40/0x50
[ 131.023034] kernfs_fop_write+0x130/0x1b0
[ 131.023034] __vfs_write+0x23/0x130
[ 131.023034] ? rcu_read_lock_sched_held+0x6d/0x80
[ 131.023034] ? rcu_sync_lockdep_assert+0x2a/0x50
[ 131.023034] ? __sb_start_write+0xd4/0x1c0
[ 131.023034] ? vfs_write+0x1a8/0x1d0
[ 131.023034] vfs_write+0xc8/0x1d0
[ 131.023034] SyS_write+0x44/0xa0
[ 131.023034] entry_SYSCALL_64_fastpath+0x1f/0xbe
[ 131.023034] RIP: 0033:0x7fb6b54ac310
[ 131.023034] RSP: 002b:00007ffcb7b123e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 131.023034] RAX: ffffffffffffffda RBX: 00007fb6b5767640 RCX: 00007fb6b54ac310
[ 131.023034] RDX: 0000000000000008 RSI: 00007fb6b5e2d000 RDI: 0000000000000001
[ 131.023034] RBP: 0000000000000007 R08: 00007fb6b57687a0 R09: 00007fb6b5e23700
[ 131.023034] R10: 0000000000000098 R11: 0000000000000246 R12: 0000000000000007
[ 131.023034] R13: 000000000173e9f0 R14: 0000000000000000 R15: 0000000000491569


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