Re: [BUG REPORT] NULL pointer dereference in jdb2_journal_grab_journal_head (RDI)

From: Jeff Merkey
Date: Mon Jan 25 2016 - 10:41:41 EST

On 1/25/16, Jan Kara <jack@xxxxxxx> wrote:
> On Sat 23-01-16 09:42:52, Jeff Merkey wrote:
>> If I leave the system in the debugger console overnight with all the
>> processors suspended for about 8 hours, then type go, the following
>> bug shows up during file I/O. This particular bug showed up while
>> using git to update some branches.
>> I have only seen this bug once and I attempted to reproduce it to get
>> a trace dump but have not been able to trigger it again. The NULL
>> pointer is RDI set to NULL while trying to obtain a lock.
>> (2)> .z grab_journal
>> ffffffffa00bb740 t jbd2_journal_grab_journal_head [jbd2]
>> (2)> u ffffffffa00bb740
>> jbd2|jbd2_journal_grab_journal_head:
>> 0xffffffffa00bb740 0F1F440000 nop DWORD PTR [rax+rax]=0x0
>> 0xffffffffa00bb745 55 push rbp
>> 0xffffffffa00bb746 4889E5 mov rbp,rsp
>> <<<<<<<<<<<< Crashes here with RDI set to NULL
>> 0xffffffffa00bb749 F00FBA2F18 lock bts DWORD PTR [rdi]=0x0,0x18
>> <<<<<<<<<<<<
> Thanks for report. Ok, this means jbd2_journal_grab_journal_head() got
> called with 'bh == NULL'. That is certainly wrong but unless we know a full
> stack trace, it's hard to guess what went wrong.
> Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

I have a system setup to get a better trace if it triggers again.
When it happened the first time I was not able to get a good dump. If
it triggers again, I'll send you the trace. So far it has not