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BUG: KCSAN: data-race in ext4_nonda_switch / percpu_counter_add_batch

write to 0xffff8881265b8958 of 8 bytes by task 12726 on cpu 0:
 percpu_counter_add_batch+0xbc/0x140 lib/percpu_counter.c:91
 percpu_counter_add include/linux/percpu_counter.h:55 [inline]
 ext4_claim_free_clusters+0x64/0x80 fs/ext4/balloc.c:602
 ext4_mb_new_blocks+0x18a/0x1ea0 fs/ext4/mballoc.c:4494
 ext4_ext_map_blocks+0x1a5c/0x20f0 fs/ext4/extents.c:4440
 ext4_map_blocks+0x230/0xcf0 fs/ext4/inode.c:622
 ext4_alloc_file_blocks+0x22f/0x6f0 fs/ext4/extents.c:4629
 ext4_fallocate+0x9ec/0x2240 fs/ext4/extents.c:4910
 vfs_fallocate+0x3b1/0x650 fs/open.c:309
 ioctl_preallocate+0x12a/0x190 fs/ioctl.c:504
 file_ioctl fs/ioctl.c:544 [inline]
 do_vfs_ioctl+0xa6b/0xd00 fs/ioctl.c:742
 ksys_ioctl+0xb2/0x150 fs/ioctl.c:761
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881265b8958 of 8 bytes by task 12724 on cpu 1:
 ext4_nonda_switch+0x58/0x130 fs/ext4/inode.c:2893
 ext4_da_write_begin+0xcc/0x860 fs/ext4/inode.c:2939
 generic_perform_write+0x13a/0x320 mm/filemap.c:3287
 ext4_buffered_write_iter+0x14e/0x280 fs/ext4/file.c:270
 ext4_file_write_iter+0xf4/0xd30 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1901 [inline]
 do_iter_readv_writev+0x4a7/0x5d0 fs/read_write.c:693
 do_iter_write fs/read_write.c:998 [inline]
 do_iter_write+0x137/0x3a0 fs/read_write.c:979
 vfs_iter_write+0x56/0x80 fs/read_write.c:1039
 iter_file_splice_write+0x530/0x830 fs/splice.c:760
 do_splice_from fs/splice.c:863 [inline]
 direct_splice_actor+0x97/0xb0 fs/splice.c:1037
 splice_direct_to_actor+0x22f/0x540 fs/splice.c:992
 do_splice_direct+0x152/0x1d0 fs/splice.c:1080
 do_sendfile+0x396/0x810 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1575 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0xb8/0x140 fs/read_write.c:1567
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12724 Comm: syz-executor.0 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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