4.3 serial driver crashes with console shortly after boot

From: Andi Kleen
Date: Mon Nov 09 2015 - 19:29:50 EST


With 4.3 an x86 server is always crashing roughly a minute after boot
in __uart_start/uart_tx_stopped. This is repeatable over multiple boots.

The back trace is
commit_echoes-> uart_flush_chars->uart_start

It seems to follow a bad pointer here

ffffffff813bbdfa: f6 80 f4 01 00 00 01 testb $0x1,0x1f4(%rax)
ffffffff813bbe01: 74 01 je ffffffff813bbe04

Unfortunately I don't have the contents of RAX which scrolled away,
but since CR2 is 1f4 I suspect it's NUL.

It seems to depend on the order of the console=... arguments on the
kernel command line. With console=tty0 console=ttyS0,115200n8 it
crashes, but when reversing the options it does not crash.

ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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