[BUG] I found a bug when try to enable record_ftrace

[Zhengyejian (Zetta)]

Hello,

There may be a deadlock caused by ftrace recursion when try to enable 
record_ftrace.
I'd like to known if the patchset 
(https://lore.kernel.org/lkml/20201106023235.367190737@xxxxxxxxxxx/) is 
trying to fix it.

Procedure for reproducing the problem is:
  1) this problem appears both in 5.13-rc4 and 5.10;

  2) my work environment is:
    qemu-arm version 4.0.0
    arm-linux-gnueabi-gcc (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04) 7.5.0

  3) then try to enable record_ftrace:
    / # mount -t pstore pstore /sys/fs/pstore
    / # echo 1 > /sys/kernel/debug/pstore/record_ftrace

  4) then system appears to be stuck, and use arm-linux-gnueabi-gdb 
dump the following call stack:
    #0  arch_spin_lock (lock=0x811a0f98) at 
/home/zyj/Linux/linux-master/arch/arm/include/asm/spinlock.h:74
    #1  do_raw_spin_lock_flags (flags=<synthetic pointer>, 
lock=0x811a0f98) at 
/home/zyj/Linux/linux-master/include/linux/spinlock.h:195
    #2  __raw_spin_lock_irqsave (lock=0x811a0f98) at 
/home/zyj/Linux/linux-master/include/linux/spinlock_api_smp.h:119
    #3  _raw_spin_lock_irqsave (lock=lock@entry=0x811a0f98) at 
/home/zyj/Linux/linux-master/kernel/locking/spinlock.c:159
    #4  0x8046c6e0 in buffer_size_add (prz=prz@entry=0x811a0f80, 
a=a@entry=16) at /home/zyj/Linux/linux-master/fs/pstore/ram_core.c:82
    #5  0x8046cc20 in persistent_ram_write (prz=0x811a0f80, 
s=0x81137b80, count=16) at 
/home/zyj/Linux/linux-master/fs/pstore/ram_core.c:327
    #6  0x8046b438 in ramoops_pstore_write (record=0x81137b90) at 
/home/zyj/Linux/linux-master/fs/pstore/ram.c:331
    #7  0x8046add8 in pstore_ftrace_call (ip=2156609456, 
parent_ip=2152122068, op=<optimized out>, fregs=<optimized out>)
        at /home/zyj/Linux/linux-master/fs/pstore/ftrace.c:54
    #8  0x801dd580 in __ftrace_ops_list_func (ignored=0x0, fregs=0x0, 
parent_ip=2152122068, ip=2156609456) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:7003
    #9  ftrace_ops_list_func (ip=2156609456, parent_ip=2152122068, 
op=<optimized out>, fregs=0x0) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:7028
    #10 0x801109f8 in ftrace_caller () at 
/home/zyj/Linux/linux-master/arch/arm/kernel/entry-ftrace.S:224
    #11 0x808b3fb4 in _raw_spin_unlock_irqrestore 
(lock=lock@entry=0x811a0f98, flags=1610612883) at 
/home/zyj/Linux/linux-master/kernel/locking/spinlock.c:190
    #12 0x8046c6d4 in buffer_size_add (prz=prz@entry=0x811a0f80, 
a=a@entry=16) at /home/zyj/Linux/linux-master/fs/pstore/ram_core.c:95
    #13 0x8046cc20 in persistent_ram_write (prz=0x811a0f80, 
s=0x81137cf8, count=16) at 
/home/zyj/Linux/linux-master/fs/pstore/ram_core.c:327
    #14 0x8046b438 in ramoops_pstore_write (record=0x81137d08) at 
/home/zyj/Linux/linux-master/fs/pstore/ram.c:331
    #15 0x8046add8 in pstore_ftrace_call (ip=2148632188, 
parent_ip=2148601660, op=<optimized out>, fregs=<optimized out>)
        at /home/zyj/Linux/linux-master/fs/pstore/ftrace.c:54
    #16 0x801dd580 in __ftrace_ops_list_func (ignored=0x0, fregs=0x0, 
parent_ip=2148601660, ip=2148632188) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:7003
    #17 ftrace_ops_list_func (ip=2148632188, parent_ip=2148601660, 
op=<optimized out>, fregs=0x0) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:7028
    #18 0x801109f8 in ftrace_caller () at 
/home/zyj/Linux/linux-master/arch/arm/kernel/entry-ftrace.S:224
    #19 0x80118680 in __set_fixmap (idx=idx@entry=FIX_TEXT_POKE0, 
phys=phys@entry=0, prot=prot@entry=0) at 
/home/zyj/Linux/linux-master/arch/arm/mm/mmu.c:385
    #20 0x80110f3c in patch_unmap (flags=<synthetic pointer>, 
fixmap=129) at /home/zyj/Linux/linux-master/arch/arm/kernel/patch.c:45
    #21 __patch_text_real (addr=addr@entry=0x808b3fb0 
<_raw_spin_unlock_irqrestore+16>, insn=insn@entry=3957420680, 
remap=remap@entry=true)
        at /home/zyj/Linux/linux-master/arch/arm/kernel/patch.c:104
    #22 0x80110b40 in __patch_text (insn=3957420680, addr=0x808b3fb0 
<_raw_spin_unlock_irqrestore+16>) at 
/home/zyj/Linux/linux-master/arch/arm/include/asm/patch.h:10
    #23 ftrace_modify_code (pc=2156609456, old=3904716800, 
new=3957420680, validate=true) at 
/home/zyj/Linux/linux-master/arch/arm/kernel/ftrace.c:95
    #24 0x80110cf4 in ftrace_make_call (rec=rec@entry=0x81007ce0, 
addr=addr@entry=2148600280) at 
/home/zyj/Linux/linux-master/arch/arm/kernel/ftrace.c:132
    #25 0x801dda40 in __ftrace_replace_code (rec=rec@entry=0x81007ce0, 
enable=enable@entry=true) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:2524
    #26 0x801de17c in ftrace_replace_code (mod_flags=mod_flags@entry=1) 
at /home/zyj/Linux/linux-master/kernel/trace/ftrace.c:2554
    #27 0x801de39c in ftrace_modify_all_code (command=5) at 
/home/zyj/Linux/linux-master/kernel/trace/ftrace.c:2698
    #28 0x80110ad8 in __ftrace_modify_code (data=<optimized out>) at 
/home/zyj/Linux/linux-master/arch/arm/kernel/ftrace.c:39
    #29 0x801d4da4 in multi_cpu_stop (data=data@entry=0x81c69d84) at 
/home/zyj/Linux/linux-master/kernel/stop_machine.c:240
    #30 0x801d4a14 in cpu_stopper_thread (cpu=<optimized out>) at 
/home/zyj/Linux/linux-master/kernel/stop_machine.c:511
    #31 0x8014de3c in smpboot_thread_fn (data=0x810c1c80) at 
/home/zyj/Linux/linux-master/kernel/smpboot.c:165
    #32 0x8014a0dc in kthread (_create=0x810c1d40) at 
/home/zyj/Linux/linux-master/kernel/kthread.c:313
    #33 0x80100150 in ret_from_fork () at 
/home/zyj/Linux/linux-master/arch/arm/kernel/entry-common.S:158
    Backtrace stopped: previous frame identical to this frame (corrupt 
stack?)

See above #7~#15, there is a recursion in function pstore_ftrace_call(), 
and a spin_lock(lock=0x811a0f98) is hold since unlock operation in above 
#11 not finished. Then in above #0, trying to acquire same lock cause a 
deadlock.

Enabling 'record_ftrace' seems a basic operation of pstore/ftrace, Does 
it mean this feature is not available for a while?

Best Regards,
Zheng