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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 13 Apr 2020 16:19:46 -0700
Message-ID: <CAHbLzkoTC3WoNa-dLBBmi15oBRXNbJzJuS6-GPr8XPWiHrgO4A@mail.gmail.com>
Subject: Re: possible deadlock in shmem_mfill_atomic_pte
To:     syzbot <syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, syzkaller-bugs@googlegroups.com
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Sender: linux-kernel-owner@vger.kernel.org
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On Tue, Mar 31, 2020 at 10:21 AM syzbot
<syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    527630fb Merge tag 'clk-fixes-for-linus' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1214875be00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=27392dd2975fd692
> dashboard link: https://syzkaller.appspot.com/bug?extid=e27980339d305f2dbfd9
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+e27980339d305f2dbfd9@syzkaller.appspotmail.com
>
> WARNING: possible irq lock inversion dependency detected
> 5.6.0-rc7-syzkaller #0 Not tainted
> --------------------------------------------------------
> syz-executor.0/10317 just changed the state of lock:
> ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
> ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
> but this lock was taken by another, SOFTIRQ-safe lock in the past:
>  (&(&xa->xa_lock)->rlock#5){..-.}
>
>
> and interrupts could create inverse lock ordering between them.
>
>
> other info that might help us debug this:
>  Possible interrupt unsafe locking scenario:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&(&info->lock)->rlock);
>                                local_irq_disable();
>                                lock(&(&xa->xa_lock)->rlock#5);
>                                lock(&(&info->lock)->rlock);
>   <Interrupt>
>     lock(&(&xa->xa_lock)->rlock#5);
>
>  *** DEADLOCK ***

This looks possible. shmem_mfill_atomic_pte() acquires info->lock with
irq enabled.

The below patch should be able to fix it:

diff --git a/mm/shmem.c b/mm/shmem.c
index d722eb8..762da6a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2399,11 +2399,11 @@ static int shmem_mfill_atomic_pte(struct
mm_struct *dst_mm,

        lru_cache_add_anon(page);

-       spin_lock(&info->lock);
+       spin_lock_irq(&info->lock);
        info->alloced++;
        inode->i_blocks += BLOCKS_PER_PAGE;
        shmem_recalc_inode(inode);
-       spin_unlock(&info->lock);
+       spin_unlock_irq(&info->lock);

        inc_mm_counter(dst_mm, mm_counter_file(page));
        page_add_file_rmap(page, false);


>
> 2 locks held by syz-executor.0/10317:
>  #0: ffff888011721898 (&mm->mmap_sem#2){++++}, at: __mcopy_atomic mm/userfaultfd.c:474 [inline]
>  #0: ffff888011721898 (&mm->mmap_sem#2){++++}, at: mcopy_atomic+0x185/0x2510 mm/userfaultfd.c:607
>  #1: ffff888024eda280 (&(ptlock_ptr(page))->rlock#2){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
>  #1: ffff888024eda280 (&(ptlock_ptr(page))->rlock#2){+.+.}, at: shmem_mfill_atomic_pte+0xf76/0x21c0 mm/shmem.c:2394
>
> the shortest dependencies between 2nd lock and 1st lock:
>  -> (&(&xa->xa_lock)->rlock#5){..-.} {
>     IN-SOFTIRQ-W at:
>                       lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>                       __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>                       _raw_spin_lock_irqsave+0x8c/0xbf kernel/locking/spinlock.c:159
>                       test_clear_page_writeback+0x1d7/0x11e0 mm/page-writeback.c:2728
>                       end_page_writeback+0x239/0x520 mm/filemap.c:1317
>                       end_buffer_async_write+0x6a9/0xa30 fs/buffer.c:389
>                       end_bio_bh_io_sync+0xe2/0x140 fs/buffer.c:3018
>                       bio_endio+0x473/0x820 block/bio.c:1872
>                       req_bio_endio block/blk-core.c:245 [inline]
>                       blk_update_request+0x3e1/0xdc0 block/blk-core.c:1468
>                       scsi_end_request+0x80/0x7a0 drivers/scsi/scsi_lib.c:576
>                       scsi_io_completion+0x1e7/0x1300 drivers/scsi/scsi_lib.c:960
>                       scsi_softirq_done+0x31e/0x3b0 drivers/scsi/scsi_lib.c:1476
>                       blk_done_softirq+0x2db/0x440 block/blk-softirq.c:37
>                       __do_softirq+0x26c/0x99d kernel/softirq.c:292
>                       invoke_softirq kernel/softirq.c:373 [inline]
>                       irq_exit+0x192/0x1d0 kernel/softirq.c:413
>                       exiting_irq arch/x86/include/asm/apic.h:546 [inline]
>                       do_IRQ+0xde/0x280 arch/x86/kernel/irq.c:263
>                       ret_from_intr+0x0/0x36
>                       clear_page_erms+0x7/0x10 arch/x86/lib/clear_page_64.S:48
>                       clear_page arch/x86/include/asm/page_64.h:49 [inline]
>                       clear_highpage include/linux/highmem.h:214 [inline]
>                       kernel_init_free_pages+0x92/0x120 mm/page_alloc.c:1118
>                       prep_new_page+0x12e/0x1f0 mm/page_alloc.c:2160
>                       get_page_from_freelist+0x14c7/0x3ee0 mm/page_alloc.c:3684
>                       __alloc_pages_nodemask+0x2a5/0x820 mm/page_alloc.c:4731
>                       alloc_pages_current+0xff/0x200 mm/mempolicy.c:2211
>                       alloc_pages include/linux/gfp.h:532 [inline]
>                       __page_cache_alloc+0x298/0x480 mm/filemap.c:959
>                       __do_page_cache_readahead+0x1a7/0x570 mm/readahead.c:196
>                       ra_submit mm/internal.h:62 [inline]
>                       ondemand_readahead+0x566/0xd60 mm/readahead.c:492
>                       page_cache_async_readahead mm/readahead.c:574 [inline]
>                       page_cache_async_readahead+0x43d/0x7c0 mm/readahead.c:547
>                       generic_file_buffered_read mm/filemap.c:2037 [inline]
>                       generic_file_read_iter+0x124a/0x2b00 mm/filemap.c:2302
>                       ext4_file_read_iter fs/ext4/file.c:131 [inline]
>                       ext4_file_read_iter+0x1d1/0x600 fs/ext4/file.c:114
>                       call_read_iter include/linux/fs.h:1896 [inline]
>                       new_sync_read+0x4a2/0x790 fs/read_write.c:414
>                       __vfs_read+0xc9/0x100 fs/read_write.c:427
>                       integrity_kernel_read+0x143/0x200 security/integrity/iint.c:200
>                       ima_calc_file_hash_tfm+0x2aa/0x3b0 security/integrity/ima/ima_crypto.c:360
>                       ima_calc_file_shash security/integrity/ima/ima_crypto.c:391 [inline]
>                       ima_calc_file_hash+0x199/0x540 security/integrity/ima/ima_crypto.c:456
>                       ima_collect_measurement+0x4c4/0x570 security/integrity/ima/ima_api.c:249
>                       process_measurement+0xc6d/0x1740 security/integrity/ima/ima_main.c:326
>                       ima_bprm_check+0xde/0x210 security/integrity/ima/ima_main.c:417
>                       security_bprm_check+0x89/0xb0 security/security.c:819
>                       search_binary_handler+0x70/0x580 fs/exec.c:1649
>                       exec_binprm fs/exec.c:1705 [inline]
>                       __do_execve_file.isra.0+0x12fc/0x2270 fs/exec.c:1825
>                       do_execveat_common fs/exec.c:1871 [inline]
>                       do_execve fs/exec.c:1888 [inline]
>                       __do_sys_execve fs/exec.c:1964 [inline]
>                       __se_sys_execve fs/exec.c:1959 [inline]
>                       __x64_sys_execve+0x8a/0xb0 fs/exec.c:1959
>                       do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>                       entry_SYSCALL_64_after_hwframe+0x49/0xbe
>     INITIAL USE at:
>                      lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>                      __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]
>                      _raw_spin_lock_irq+0x5b/0x80 kernel/locking/spinlock.c:167
>                      spin_lock_irq include/linux/spinlock.h:363 [inline]
>                      clear_inode+0x1b/0x1e0 fs/inode.c:529
>                      shmem_evict_inode+0x1db/0x9f0 mm/shmem.c:1116
>                      evict+0x2ed/0x650 fs/inode.c:576
>                      iput_final fs/inode.c:1572 [inline]
>                      iput+0x536/0x8c0 fs/inode.c:1598
>                      dentry_unlink_inode+0x2c0/0x3e0 fs/dcache.c:374
>                      d_delete fs/dcache.c:2451 [inline]
>                      d_delete+0x117/0x150 fs/dcache.c:2440
>                      vfs_unlink+0x4d5/0x620 fs/namei.c:4087
>                      handle_remove+0x417/0x720 drivers/base/devtmpfs.c:332
>                      handle drivers/base/devtmpfs.c:378 [inline]
>                      devtmpfsd.part.0+0x302/0x750 drivers/base/devtmpfs.c:413
>                      devtmpfsd+0x107/0x120 drivers/base/devtmpfs.c:403
>                      kthread+0x357/0x430 kernel/kthread.c:255
>                      ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
>   }
>   ... key      at: [<ffffffff8c402aa0>] __key.17910+0x0/0x40
>   ... acquired at:
>    __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>    _raw_spin_lock_irqsave+0x8c/0xbf kernel/locking/spinlock.c:159
>    shmem_uncharge+0x24/0x270 mm/shmem.c:341
>    __split_huge_page mm/huge_memory.c:2540 [inline]
>    split_huge_page_to_list+0x2751/0x33c0 mm/huge_memory.c:2813
>    split_huge_page include/linux/huge_mm.h:169 [inline]
>    shmem_unused_huge_shrink+0x7ba/0x13a0 mm/shmem.c:542
>    shmem_unused_huge_scan+0x7a/0xb0 mm/shmem.c:574
>    super_cache_scan+0x34f/0x480 fs/super.c:111
>    do_shrink_slab+0x3fc/0xab0 mm/vmscan.c:512
>    shrink_slab mm/vmscan.c:673 [inline]
>    shrink_slab+0x16f/0x5f0 mm/vmscan.c:646
>    shrink_node_memcgs mm/vmscan.c:2676 [inline]
>    shrink_node+0x477/0x1b20 mm/vmscan.c:2780
>    shrink_zones mm/vmscan.c:2983 [inline]
>    do_try_to_free_pages+0x38d/0x13a0 mm/vmscan.c:3036
>    try_to_free_pages+0x293/0x8d0 mm/vmscan.c:3275
>    __perform_reclaim mm/page_alloc.c:4113 [inline]
>    __alloc_pages_direct_reclaim mm/page_alloc.c:4134 [inline]
>    __alloc_pages_slowpath+0x919/0x26a0 mm/page_alloc.c:4537
>    __alloc_pages_nodemask+0x5e1/0x820 mm/page_alloc.c:4751
>    __alloc_pages include/linux/gfp.h:496 [inline]
>    __alloc_pages_node include/linux/gfp.h:509 [inline]
>    alloc_pages_vma+0x3bd/0x600 mm/mempolicy.c:2155
>    shmem_alloc_hugepage+0x122/0x210 mm/shmem.c:1484
>    shmem_alloc_and_acct_page+0x3ba/0x980 mm/shmem.c:1522
>    shmem_getpage_gfp+0xdb9/0x2860 mm/shmem.c:1835
>    shmem_getpage mm/shmem.c:154 [inline]
>    shmem_write_begin+0x102/0x1e0 mm/shmem.c:2488
>    generic_perform_write+0x20a/0x4e0 mm/filemap.c:3287
>    __generic_file_write_iter+0x24c/0x610 mm/filemap.c:3416
>    generic_file_write_iter+0x3f0/0x62d mm/filemap.c:3448
>    call_write_iter include/linux/fs.h:1902 [inline]
>    new_sync_write+0x49c/0x700 fs/read_write.c:483
>    __vfs_write+0xc9/0x100 fs/read_write.c:496
>    vfs_write+0x262/0x5c0 fs/read_write.c:558
>    ksys_write+0x127/0x250 fs/read_write.c:611
>    do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> (&(&info->lock)->rlock){+.+.} {
>    HARDIRQ-ON-W at:
>                     lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>                     __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>                     _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>                     spin_lock include/linux/spinlock.h:338 [inline]
>                     shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
>                     shmem_mcopy_atomic_pte+0x3a/0x50 mm/shmem.c:2445
>                     mfill_atomic_pte mm/userfaultfd.c:434 [inline]
>                     __mcopy_atomic mm/userfaultfd.c:557 [inline]
>                     mcopy_atomic+0xac7/0x2510 mm/userfaultfd.c:607
>                     userfaultfd_copy fs/userfaultfd.c:1736 [inline]
>                     userfaultfd_ioctl+0x4d2/0x3b10 fs/userfaultfd.c:1886
>                     vfs_ioctl fs/ioctl.c:47 [inline]
>                     ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
>                     __do_sys_ioctl fs/ioctl.c:772 [inline]
>                     __se_sys_ioctl fs/ioctl.c:770 [inline]
>                     __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
>                     do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>                     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>    SOFTIRQ-ON-W at:
>                     lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>                     __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>                     _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>                     spin_lock include/linux/spinlock.h:338 [inline]
>                     shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
>                     shmem_mcopy_atomic_pte+0x3a/0x50 mm/shmem.c:2445
>                     mfill_atomic_pte mm/userfaultfd.c:434 [inline]
>                     __mcopy_atomic mm/userfaultfd.c:557 [inline]
>                     mcopy_atomic+0xac7/0x2510 mm/userfaultfd.c:607
>                     userfaultfd_copy fs/userfaultfd.c:1736 [inline]
>                     userfaultfd_ioctl+0x4d2/0x3b10 fs/userfaultfd.c:1886
>                     vfs_ioctl fs/ioctl.c:47 [inline]
>                     ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
>                     __do_sys_ioctl fs/ioctl.c:772 [inline]
>                     __se_sys_ioctl fs/ioctl.c:770 [inline]
>                     __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
>                     do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>                     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>    INITIAL USE at:
>                    lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>                    __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]
>                    _raw_spin_lock_irq+0x5b/0x80 kernel/locking/spinlock.c:167
>                    spin_lock_irq include/linux/spinlock.h:363 [inline]
>                    shmem_getpage_gfp+0xf10/0x2860 mm/shmem.c:1887
>                    shmem_read_mapping_page_gfp+0xd3/0x170 mm/shmem.c:4218
>                    shmem_read_mapping_page include/linux/shmem_fs.h:101 [inline]
>                    drm_gem_get_pages+0x293/0x530 drivers/gpu/drm/drm_gem.c:578
>                    drm_gem_shmem_get_pages_locked drivers/gpu/drm/drm_gem_shmem_helper.c:146 [inline]
>                    drm_gem_shmem_get_pages+0x9d/0x160 drivers/gpu/drm/drm_gem_shmem_helper.c:175
>                    virtio_gpu_object_attach+0x121/0x950 drivers/gpu/drm/virtio/virtgpu_vq.c:1090
>                    virtio_gpu_object_create+0x26f/0x490 drivers/gpu/drm/virtio/virtgpu_object.c:150
>                    virtio_gpu_gem_create+0xaa/0x1d0 drivers/gpu/drm/virtio/virtgpu_gem.c:42
>                    virtio_gpu_mode_dumb_create+0x21e/0x360 drivers/gpu/drm/virtio/virtgpu_gem.c:82
>                    drm_mode_create_dumb+0x27c/0x300 drivers/gpu/drm/drm_dumb_buffers.c:94
>                    drm_client_buffer_create drivers/gpu/drm/drm_client.c:267 [inline]
>                    drm_client_framebuffer_create+0x1b7/0x770 drivers/gpu/drm/drm_client.c:412
>                    drm_fb_helper_generic_probe+0x1e4/0x810 drivers/gpu/drm/drm_fb_helper.c:2051
>                    drm_fb_helper_single_fb_probe drivers/gpu/drm/drm_fb_helper.c:1600 [inline]
>                    __drm_fb_helper_initial_config_and_unlock+0xb56/0x11e0 drivers/gpu/drm/drm_fb_helper.c:1758
>                    drm_fb_helper_initial_config drivers/gpu/drm/drm_fb_helper.c:1853 [inline]
>                    drm_fb_helper_initial_config drivers/gpu/drm/drm_fb_helper.c:1845 [inline]
>                    drm_fbdev_client_hotplug+0x30f/0x580 drivers/gpu/drm/drm_fb_helper.c:2145
>                    drm_fbdev_generic_setup drivers/gpu/drm/drm_fb_helper.c:2224 [inline]
>                    drm_fbdev_generic_setup+0x18b/0x295 drivers/gpu/drm/drm_fb_helper.c:2197
>                    virtio_gpu_probe+0x28f/0x2de drivers/gpu/drm/virtio/virtgpu_drv.c:126
>                    virtio_dev_probe+0x463/0x710 drivers/virtio/virtio.c:248
>                    really_probe+0x281/0x6d0 drivers/base/dd.c:551
>                    driver_probe_device+0x104/0x210 drivers/base/dd.c:724
>                    device_driver_attach+0x108/0x140 drivers/base/dd.c:998
>                    __driver_attach+0xda/0x240 drivers/base/dd.c:1075
>                    bus_for_each_dev+0x14b/0x1d0 drivers/base/bus.c:305
>                    bus_add_driver+0x4a2/0x5a0 drivers/base/bus.c:622
>                    driver_register+0x1c4/0x330 drivers/base/driver.c:171
>                    do_one_initcall+0x10a/0x7d0 init/main.c:1152
>                    do_initcall_level init/main.c:1225 [inline]
>                    do_initcalls init/main.c:1241 [inline]
>                    do_basic_setup init/main.c:1261 [inline]
>                    kernel_init_freeable+0x501/0x5ae init/main.c:1445
>                    kernel_init+0xd/0x1bb init/main.c:1352
>                    ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
>  }
>  ... key      at: [<ffffffff8c3f0420>] __key.55978+0x0/0x40
>  ... acquired at:
>    mark_lock_irq kernel/locking/lockdep.c:3316 [inline]
>    mark_lock+0x50e/0x1220 kernel/locking/lockdep.c:3665
>    mark_usage kernel/locking/lockdep.c:3583 [inline]
>    __lock_acquire+0x1236/0x3ca0 kernel/locking/lockdep.c:3908
>    lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>    __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>    _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>    spin_lock include/linux/spinlock.h:338 [inline]
>    shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
>    shmem_mcopy_atomic_pte+0x3a/0x50 mm/shmem.c:2445
>    mfill_atomic_pte mm/userfaultfd.c:434 [inline]
>    __mcopy_atomic mm/userfaultfd.c:557 [inline]
>    mcopy_atomic+0xac7/0x2510 mm/userfaultfd.c:607
>    userfaultfd_copy fs/userfaultfd.c:1736 [inline]
>    userfaultfd_ioctl+0x4d2/0x3b10 fs/userfaultfd.c:1886
>    vfs_ioctl fs/ioctl.c:47 [inline]
>    ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
>    __do_sys_ioctl fs/ioctl.c:772 [inline]
>    __se_sys_ioctl fs/ioctl.c:770 [inline]
>    __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
>    do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
>
> stack backtrace:
> CPU: 0 PID: 10317 Comm: syz-executor.0 Not tainted 5.6.0-rc7-syzkaller #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x188/0x20d lib/dump_stack.c:118
>  print_irq_inversion_bug kernel/locking/lockdep.c:3179 [inline]
>  check_usage_backwards.cold+0x1d/0x26 kernel/locking/lockdep.c:3230
>  mark_lock_irq kernel/locking/lockdep.c:3316 [inline]
>  mark_lock+0x50e/0x1220 kernel/locking/lockdep.c:3665
>  mark_usage kernel/locking/lockdep.c:3583 [inline]
>  __lock_acquire+0x1236/0x3ca0 kernel/locking/lockdep.c:3908
>  lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4484
>  __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>  _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>  spin_lock include/linux/spinlock.h:338 [inline]
>  shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
>  shmem_mcopy_atomic_pte+0x3a/0x50 mm/shmem.c:2445
>  mfill_atomic_pte mm/userfaultfd.c:434 [inline]
>  __mcopy_atomic mm/userfaultfd.c:557 [inline]
>  mcopy_atomic+0xac7/0x2510 mm/userfaultfd.c:607
>  userfaultfd_copy fs/userfaultfd.c:1736 [inline]
>  userfaultfd_ioctl+0x4d2/0x3b10 fs/userfaultfd.c:1886
>  vfs_ioctl fs/ioctl.c:47 [inline]
>  ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
>  __do_sys_ioctl fs/ioctl.c:772 [inline]
>  __se_sys_ioctl fs/ioctl.c:770 [inline]
>  __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
>  do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x45c6e9
> Code: bd b1 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 8b b1 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007fcff1e50c88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 000000000072bf00 RCX: 000000000045c6e9
> RDX: 00000000200a0fe0 RSI: 00000000c028aa03 RDI: 0000000000000003
> RBP: 00007fcff1e516d4 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 00000000000005b3 R14: 00000000004af2cd R15: 00000000006ec420
>
>
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>