Hi Eric,

On Wed, Dec 08, 2021 at 02:25:31PM -0600, Eric W. Biederman wrote:

Today the rules are a bit iffy and arbitrary about which kernel
threads have struct kthread present.  Both idle threads and thread
started with create_kthread want struct kthread present so that is
effectively all kernel threads.  Make the rule that if PF_KTHREAD
and the task is running then struct kthread is present.

This will allow the kernel thread code to using tsk->exit_code
with different semantics from ordinary processes.

To make ensure that struct kthread is present for all
kernel threads move it's allocation into copy_process.

Add a deallocation of struct kthread in exec for processes
that were kernel threads.

Move the allocation of struct kthread for the initial thread
earlier so that it is not repeated for each additional idle
thread.

Move the initialization of struct kthread into set_kthread_struct
so that the structure is always and reliably initailized.

Clear set_child_tid in free_kthread_struct to ensure the kthread
struct is reliably freed during exec.  The function
free_kthread_struct does not need to clear vfork_done during exec as
exec_mm_release called from exec_mmap has already cleared vfork_done.

Signed-off-by: "Eric W. Biederman" <redacted>

This patch as commit 40966e316f86 ("kthread: Ensure struct kthread is
present for all kthreads") in -next causes an ARCH=arm
multi_v5_defconfig kernel to fail to boot in QEMU. I had to apply commit
6692c98c7df5 ("fork: Stop protecting back_fork_cleanup_cgroup_lock with
CONFIG_NUMA") to get it to build and I applied commit dd621ee0cf8e
("kthread: Warn about failed allocations for the init kthread") to avoid
the known runtime warning.

$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- distclean multi_v5_defconfig all

$ qemu-system-arm \
    -initrd rootfs.cpio \
    -append earlycon \
    -machine palmetto-bmc \
    -no-reboot \
    -dtb arch/arm/boot/dts/aspeed-bmc-opp-palmetto.dtb \
    -display none \
    -kernel arch/arm/boot/zImage \
    -m 512m \
    -nodefaults \
    -serial mon:stdio
qemu-system-arm: warning: nic ftgmac100.0 has no peer
qemu-system-arm: warning: nic ftgmac100.1 has no peer
Booting Linux on physical CPU 0x0
Linux version 5.16.0-rc1-00016-g40966e316f86-dirty (nathan@archlinux-ax161) (arm-linux-gnueabi-gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2) #1 PREEMPT Wed Dec 22 18:08:53 UTC 2021
CPU: ARM926EJ-S [41069265] revision 5 (ARMv5TEJ), cr=00093177
CPU: VIVT data cache, VIVT instruction cache
OF: fdt: Machine model: Palmetto BMC
earlycon: ns16550a0 at MMIO 0x1e784000 (options '')
printk: bootconsole [ns16550a0] enabled
Memory policy: Data cache writethrough
cma: Reserved 16 MiB at 0x5b000000
Zone ranges:
  DMA      [mem 0x0000000040000000-0x000000005edfffff]
  Normal   empty
  HighMem  [mem 0x000000005ee00000-0x000000005fffffff]
Movable zone start for each node
Early memory node ranges
  node   0: [mem 0x0000000040000000-0x000000005bffffff]
  node   0: [mem 0x000000005c000000-0x000000005dffffff]
  node   0: [mem 0x000000005e000000-0x000000005edfffff]
  node   0: [mem 0x000000005ee00000-0x000000005fffffff]
Initmem setup node 0 [mem 0x0000000040000000-0x000000005fffffff]
Built 1 zonelists, mobility grouping on.  Total pages: 130084
Kernel command line: earlycon
Dentry cache hash table entries: 65536 (order: 6, 262144 bytes, linear)
Inode-cache hash table entries: 32768 (order: 5, 131072 bytes, linear)
mem auto-init: stack:off, heap alloc:off, heap free:off
Memory: 433140K/524288K available (9628K kernel code, 2019K rwdata, 2368K rodata, 340K init, 661K bss, 74764K reserved, 16384K cma-reserved, 0K highmem)
SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
rcu: Preemptible hierarchical RCU implementation.
rcu:    RCU event tracing is enabled.
        Trampoline variant of Tasks RCU enabled.
rcu: RCU calculated value of scheduler-enlistment delay is 10 jiffies.
NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
i2c controller registered, irq 16
random: get_random_bytes called from start_kernel+0x408/0x624 with crng_init=0
clocksource: FTTMR010-TIMER2: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 79635851949 ns
sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 89478484971ns
Switching to timer-based delay loop, resolution 41ns
Console: colour dummy device 80x30
printk: console [tty0] enabled
printk: bootconsole [ns16550a0] disabled

After that, it just hangs.

The rootfs is available at https://github.com/ClangBuiltLinux/boot-utils
in the images/arm folder.

If there is any more information that I can provide or changes to test,
please let me know.

Cheers,
Nathan