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BUG: KCSAN: data-race in __ppp_xmit_process / skb_dequeue

read to 0xffff8880b73b18f0 of 4 bytes by task 12694 on cpu 1:
 ppp_send_frame drivers/net/ppp/ppp_generic.c:1599 [inline]
 __ppp_xmit_process+0x58a/0xe40 drivers/net/ppp/ppp_generic.c:1409
 ppp_xmit_process+0xd1/0x1a0 drivers/net/ppp/ppp_generic.c:1430
 ppp_write+0x1a3/0x1d0 drivers/net/ppp/ppp_generic.c:514
 __vfs_write+0x67/0xc0 fs/read_write.c:494
 __kernel_write+0xb8/0x240 fs/read_write.c:515
 write_pipe_buf+0xb6/0xf0 fs/splice.c:794
 splice_from_pipe_feed fs/splice.c:500 [inline]
 __splice_from_pipe+0x248/0x480 fs/splice.c:624
 splice_from_pipe+0xbb/0x100 fs/splice.c:659
 default_file_splice_write+0x45/0x90 fs/splice.c:806
 do_splice_from fs/splice.c:848 [inline]
 direct_splice_actor+0xa0/0xc0 fs/splice.c:1020
 splice_direct_to_actor+0x215/0x510 fs/splice.c:975
 do_splice_direct+0x161/0x1e0 fs/splice.c:1063
 do_sendfile+0x384/0x7f0 fs/read_write.c:1464
 __do_sys_sendfile64 fs/read_write.c:1525 [inline]
 __se_sys_sendfile64 fs/read_write.c:1511 [inline]
 __x64_sys_sendfile64+0x12a/0x140 fs/read_write.c:1511
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880b73b18f0 of 4 bytes by task 12690 on cpu 0:
 __skb_unlink include/linux/skbuff.h:2020 [inline]
 __skb_dequeue include/linux/skbuff.h:2040 [inline]
 skb_dequeue+0x84/0x150 net/core/skbuff.c:3040
 ppp_read+0x19c/0x400 drivers/net/ppp/ppp_generic.c:439
 do_loop_readv_writev fs/read_write.c:714 [inline]
 do_loop_readv_writev fs/read_write.c:701 [inline]
 do_iter_read+0x357/0x3d0 fs/read_write.c:935
 vfs_readv+0x9c/0xf0 fs/read_write.c:997
 do_preadv+0x131/0x1d0 fs/read_write.c:1089
 __do_sys_preadv fs/read_write.c:1139 [inline]
 __se_sys_preadv fs/read_write.c:1134 [inline]
 __x64_sys_preadv+0x61/0x80 fs/read_write.c:1134
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 12690 Comm: syz-executor.4 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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