Re: Stalls when starting a VSOCK listening socket: soft lockups, RCU stalls, timeout

Jiri Slaby
2026-03-05T12:20:44Z

On 05. 03. 26, 12:53, Jiri Slaby wrote:
> On 05. 03. 26, 8:00, Jiri Slaby wrote:
>> On 02. 03. 26, 12:46, Peter Zijlstra wrote:
>>> On Mon, Mar 02, 2026 at 06:28:38AM +0100, Jiri Slaby wrote:
>>>
>>>> The state of the lock:
>>>>
>>>> crash> struct rq.__lock -x ffff8d1a6fd35dc0
>>>>    __lock = {
>>>>      raw_lock = {
>>>>        {
>>>>          val = {
>>>>            counter = 0x40003
>>>>          },
>>>>          {
>>>>            locked = 0x3,
>>>>            pending = 0x0
>>>>          },
>>>>          {
>>>>            locked_pending = 0x3,
>>>>            tail = 0x4
>>>>          }
>>>>        }
>>>>      }
>>>>    },
>>>>
>>>
>>>
>>> That had me remember the below patch that never quite made it. I've
>>> rebased it to something more recent so it applies.
>>>
>>> If you stick that in, we might get a clue as to who is owning that lock.
>>> Provided it all wants to reproduce well enough.
>>
>> Thanks, I applied it, but to date it is still not accepted yet:
>> https://build.opensuse.org/requests/1335893
>
> OK, I have a first dump with the patch applied:
>   __lock = {
>     raw_lock = {
>       {
>         val = {
>           counter = 0x2c0003
>         },
>         {
>           locked = 0x3,
>           pending = 0x0
>         },
>         {
>           locked_pending = 0x3,
>           tail = 0x2c
>         }
>       }
>     }
>   },
>
> I am not sure if it is of any help?
>
>
>
>
> BUT: I have another dump with LOCKDEP (but NOT the patch above). The
> kernel is again spinning in mm_get_cid(), presumably waiting for a free
> bit in the map as before [1]:
>
>
> [  162.660584] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> ...
> [  162.661378] Sending NMI from CPU 3 to CPUs 1:
> [  162.661398] NMI backtrace for cpu 1
> ...
> [  162.661411] RIP: 0010:mm_get_cid+0x54/0xc0
>
>
> 7680 is active on CPU 1:
> PID: 7680     TASK: ffff8cc4038525c0  CPU: 1    COMMAND: "asm"
>
>
> CPU3 is waiting for the CPU1's rq_lock:
> RDX: 0000000000000000  RSI: 0000000000000003  RDI: ffff8cc72fcb8500
> ...
>  #3 [ffffd2e9c0083da0] raw_spin_rq_lock_nested+0x20 at ffffffff9339e700
>
> crash> struct rq.__lock -x ffff8cc72fcb8500
>   __lock = {
>     raw_lock = {
>       {
>         val = {
>           counter = 0x100003
>         },
>         {
>           locked = 0x3,
>           pending = 0x0
>         },
>         {
>           locked_pending = 0x3,
>           tail = 0x10
>         }
>       }
>     },
>     magic = 0xdead4ead,
>     owner_cpu = 0x1,
>     owner = 0xffff8cc4038b8000,
>     dep_map = {
>       key = 0xffffffff96245970 <__key.7>,
>       class_cache = {0xffffffff9644b488 <lock_classes+10600>, 0x0},
>       name = 0xffffffff94ba3ab3 "&rq->__lock",
>       wait_type_outer = 0x0,
>       wait_type_inner = 0x2,
>       lock_type = 0x0
>     }
>   },
>
> owner_cpu is 1, owner is:
> PID: 7508     TASK: ffff8cc4038b8000  CPU: 1    COMMAND: "compile"
>
> But as you can see above, CPU1 is occupied with a different task:
> crash> bt -sxc 1
> PID: 7680     TASK: ffff8cc4038525c0  CPU: 1    COMMAND: "asm"
>
> spinning in mm_get_cid() as I wrote. See the objdump of mm_get_cid below.

You might be interested in mm_cid dumps:

====== PID 7508 (sleeping, holding the rq lock) ======

crash> task -R mm_cid -x 7508
PID: 7508 TASK: ffff8cc4038b8000 CPU: 1 COMMAND: "compile"
mm_cid = {
active = 0x1,
cid = 0x40000003
},

crash> p ((struct task_struct *)(0xffff8cc4038b8000))->mm->mm_cid|head -4
$6 = {
pcpu = 0x66222619df40,
mode = 1073741824,
max_cids = 4,

====== PID 7680 (spinning in mm_get_cid()) ======

crash> task -R mm_cid -x 7680
PID: 7680 TASK: ffff8cc4038525c0 CPU: 1 COMMAND: "asm"
mm_cid = {
active = 0x1,
cid = 0x80000000
},

crash> p ((struct task_struct *)(0xffff8cc4038b8000))->mm->mm_cid|head -4
$8 = {
pcpu = 0x66222619df40,
mode = 1073741824,
max_cids = 4,

====== per-cpu for CPU1 ======

crash> struct mm_cid_pcpu -x fffff2e9bfc89f40
struct mm_cid_pcpu {
cid = 0x40000003
}

Dump of any other's mm_cids needed?

> [1] https://bugzilla.suse.com/show_bug.cgi?id=1258936#c17
>
>
>> ffffffff8139cd40 <mm_get_cid>:
>> mm_get_cid():
>> include/linux/cpumask.h:1020
>> ffffffff8139cd40:       8b 05 9a d7 40 02       mov
>> 0x240d79a(%rip),%eax        # ffffffff837aa4e0 <nr_cpu_ids>
>> kernel/sched/sched.h:3779
>> ffffffff8139cd46:       55                      push   %rbp
>> ffffffff8139cd47:       53                      push   %rbx
>> include/linux/mm_types.h:1477
>> ffffffff8139cd48:       48 8d 9f 80 0b 00 00    lea    0xb80(%rdi),%rbx
>> kernel/sched/sched.h:3780 (discriminator 2)
>> ffffffff8139cd4f:       8b b7 0c 01 00 00       mov    0x10c(%rdi),%esi
>> include/linux/cpumask.h:1020
>> ffffffff8139cd55:       83 c0 3f                add    $0x3f,%eax
>> ffffffff8139cd58:       c1 e8 03                shr    $0x3,%eax
>> kernel/sched/sched.h:3780 (discriminator 2)
>> ffffffff8139cd5b:       48 89 f5                mov    %rsi,%rbp
>> include/linux/mm_types.h:1479 (discriminator 1)
>> ffffffff8139cd5e:       25 f8 ff ff 1f          and    $0x1ffffff8,%eax
>> include/linux/mm_types.h:1489 (discriminator 1)
>> ffffffff8139cd63:       48 8d 3c 43             lea    (%rbx,%rax,2),%rdi
>> include/linux/find.h:393
>> ffffffff8139cd67:       e8 44 d8 6e 00          call
>> ffffffff81a8a5b0 <_find_first_zero_bit>
>> kernel/sched/sched.h:3771
>> ffffffff8139cd6c:       39 e8                   cmp    %ebp,%eax
>> ffffffff8139cd6e:       73 7c                   jae
>> ffffffff8139cdec <mm_get_cid+0xac>
>> ffffffff8139cd70:       89 c1                   mov    %eax,%ecx
>> kernel/sched/sched.h:3773 (discriminator 1)
>> ffffffff8139cd72:       89 c2                   mov    %eax,%edx
>> include/linux/cpumask.h:1020
>> ffffffff8139cd74:       8b 05 66 d7 40 02       mov
>> 0x240d766(%rip),%eax        # ffffffff837aa4e0 <nr_cpu_ids>
>> ffffffff8139cd7a:       83 c0 3f                add    $0x3f,%eax
>> ffffffff8139cd7d:       c1 e8 03                shr    $0x3,%eax
>> include/linux/mm_types.h:1479 (discriminator 1)
>> ffffffff8139cd80:       25 f8 ff ff 1f          and    $0x1ffffff8,%eax
>> include/linux/mm_types.h:1489 (discriminator 1)
>> ffffffff8139cd85:       48 8d 04 43             lea    (%rbx,%rax,2),%rax
>> arch/x86/include/asm/bitops.h:136
>> ffffffff8139cd89:       f0 48 0f ab 10          lock bts %rdx,(%rax)
>> kernel/sched/sched.h:3773 (discriminator 2)
>> ffffffff8139cd8e:       73 4b                   jae
>> ffffffff8139cddb <mm_get_cid+0x9b>
>> ffffffff8139cd90:       eb 5a                   jmp
>> ffffffff8139cdec <mm_get_cid+0xac>
>> arch/x86/include/asm/vdso/processor.h:13
>> ffffffff8139cd92:       f3 90                   pause
>> include/linux/cpumask.h:1020
>> ffffffff8139cd94:       8b 05 46 d7 40 02       mov
>> 0x240d746(%rip),%eax        # ffffffff837aa4e0 <nr_cpu_ids>
>
> The CPU1 was caught by the NMI here ^^^^^^^^^^^^^^^^^^^^.
>
>
>
>
>> In the meantime, me and Michal K. did some digging into qemu dumps.
>> Details at (and a couple previous comments):
>> https://bugzilla.suse.com/show_bug.cgi?id=1258936#c17
>>
>> tl;dr:
>>
>> In one of the dumps, one process sits in
>>    context_switch
>>      -> mm_get_cid (before switch_to())
>>
>>  > 65 kworker/1:1 SP= 0xffffcf82c022fd98 -> __schedule+0x16ee
>> (ffffffff820f162e) -> call mm_get_cid
>>
>> Michal extracted the vCPU's RIP and it turned out:
>>  > Hm, I'd say the CPU could be spinning in mm_get_cid() waiting for a
>> free CID.
>>  > ...
>>  > ffff8a88458137c0:  000000000000000f 000000000000000f
>>  >                                                    ^
>>  > Hm, so indeed CIDs for all four CPUs are occupied.
>>
>> To me (I don't know what CID is either), this might point as a
>> possible culprit to Thomas' "sched/mmcid: Cure mode transition woes" [1].
>>
>> Funnily enough, 47ee94efccf6 ("sched/mmcid: Protect transition on
>> weakly ordered systems") spells:
>>  >     As a consequence the task will
>>  >     not drop the CID when scheduling out before the fixup is
>> completed, which
>>  >     means the CID space can be exhausted and the next task
>> scheduling in will
>>  >     loop in mm_get_cid() and the fixup thread can livelock on the
>> held runqueue
>>  >     lock as above.
>>
>> Which sounds like what exactly happens here. Except the patch is from
>> the series above, so is already in 6.19 obviously.
>>
>>
>> I noticed there is also a 7.0-rc1 fix:
>>    1e83ccd5921a sched/mmcid: Don't assume CID is CPU owned on mode switch
>> But that got into 6.19.1 already (we are at 6.19.3). So does not
>> improve the situation.
>>
>> Any ideas?
>>
>>
>>
>> [1] https://lore.kernel.org/all/20260201192234.380608594@kernel.org/
>>
>> thanks,
>

--
js
suse labs