Re: Linux 5.14-rc1

From: Linus Torvalds
Date: Mon Jul 12 2021 - 15:14:47 EST

On Mon, Jul 12, 2021 at 12:08 AM Jon Masters <jcm@xxxxxxxxxxxxxx> wrote:
> I happened to be installing a Fedora 34 (x86) VM for something and did a
> test kernel compile that hung on boot. Setting up a serial console I get
> the below backtrace from ttm but I have not had chance to look at it.

It's a NULL pointer in qxl_bo_delete_mem_notify(), with the code
disassembling to

16: 55 push %rbp
17: 48 89 fd mov %rdi,%rbp
1a: e8 a2 02 00 00 callq 0x2c1
1f: 84 c0 test %al,%al
21: 74 0d je 0x30
23: 48 8b 85 68 01 00 00 mov 0x168(%rbp),%rax
2a:* 83 78 10 03 cmpl $0x3,0x10(%rax) <-- trapping instruction
2e: 74 02 je 0x32
30: 5d pop %rbp
31: c3 retq

and that "cmpl $3" looks exactly like that

if (bo->resource->mem_type == TTM_PL_PRIV

and the bug is almost certainly from commit d3116756a710 ("drm/ttm:
rename bo->mem and make it a pointer"), which did

- if (bo->mem.mem_type == TTM_PL_PRIV ...
+ if (bo->resource->mem_type == TTM_PL_PRIV ...

and claimed "No functional change".

But clearly the "bo->resource" pointer is NULL.

Added guilty parties and dri-devel mailing list.

Christian? Full report at

but there's not a whole lot else there that is interesting except for
the call trace:

ttm_bo_cleanup_memtype_use+0x22/0x60 [ttm]
ttm_bo_release+0x1a1/0x300 [ttm]
ttm_bo_delayed_delete+0x1be/0x220 [ttm]
ttm_device_delayed_workqueue+0x18/0x40 [ttm]

so it's presumably the cleanup phase and perhaps "bo->resource" has
been deallocated and cleared?