[PATCH v4 9/9] riscv: Fixup lockdep_assert_held(&text_mutex) in patch_insn_write

[Guo Ren]

On Mon, Oct 19, 2020 at 4:36 PM Masami Hiramatsu <mhiramat at kernel.org> wrote:
>
> On Sat, 17 Oct 2020 07:06:17 +0000
> guoren at kernel.org wrote:
>
> > From: Guo Ren <guoren at linux.alibaba.com>
> >
> > It will cause warning messages:
> > echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > [   47.691397] ------------[ cut here ]------------
> > [   47.692899] WARNING: CPU: 0 PID: 11 at arch/riscv/kernel/patch.c:63 patch_insn_write+0x182/0x19a
> > [   47.694483] Modules linked in:
> > [   47.695754] CPU: 0 PID: 11 Comm: migration/0 Not tainted 5.9.0-11367-g1054335 #132
> > [   47.698228] epc: ffffffe000204530 ra : ffffffe00020452c sp : ffffffe0023ffc20 gp : ffffffe0013e1fe0 tp : ffffffe0023e4e00 t0 : 0000000000000000
> > [   47.701872]  t1 : 000000000000000e t2 : 000000000000001b s0 : ffffffe0023ffc70 s1 : ffffffe000206850 a0 : 0000000000000000 a1 : ffffffffffffffff
> > [   47.705550]  a2 : 0000000000000000 a3 : ffffffe03af7c5e8 a4 : 0000000000000000 a5 : 0000000000000000 a6 : 0000000000000000 a7 : 00000000150b02d8
> > [   47.709159]  s2 : 0000000000000008 s3 : 0000000000000858 s4 : ffffffe0023ffc98 s5 : 0000000000000850 s6 : 0000000000000003 s7 : 0000000000000002
> > [   47.714135]  s8 : 0000000000000004 s9 : 0000000000000001 s10: 0000000000000001 s11: 0000000000000003 t3 : ffffffffffffe000 t4 : 0000000000d86254
> > [   47.716574]  t5 : 0000000000000005 t6 : 0000000000040000status: 0000000000000100 badaddr: 0000000000000000 cause: 0000000000000003
> > [   47.720019] CPU: 0 PID: 11 Comm: migration/0 Not tainted 5.9.0-11367-g1054335 #132
> > [   47.722074] Call Trace:
> > [   47.722561] [<ffffffe000203c88>] walk_stackframe+0x0/0xc2
> > [   47.724608] [<ffffffe000b2020a>] show_stack+0x46/0x52
> > [   47.726246] [<ffffffe000b23c24>] dump_stack+0x90/0xb6
> > [   47.727672] [<ffffffe00020c9c4>] __warn+0x98/0xfa
> > [   47.729131] [<ffffffe0005f4be0>] report_bug+0xaa/0x11e
> > [   47.730624] [<ffffffe000203b74>] do_trap_break+0x96/0xfe
> > [   47.732448] [<ffffffe000201daa>] ret_from_exception+0x0/0x14
> > [   47.734341] [<ffffffe00020452c>] patch_insn_write+0x17e/0x19a
> > [   47.737235] irq event stamp: 39
> > [   47.738521] hardirqs last  enabled at (39): [<ffffffe000201d0a>] _save_context+0xa2/0xe6
> > [   47.741055] hardirqs last disabled at (38): [<ffffffe0002c2404>] multi_cpu_stop+0x130/0x166
> > [   47.743551] softirqs last  enabled at (0): [<ffffffe00020a4d2>] copy_process+0x430/0x1316
> > [   47.746031] softirqs last disabled at (0): [<0000000000000000>] 0x0
> > [   47.748617] ---[ end trace 88a1054faa6524ef ]---
> >
> > Because the path of stop_machine(__ftrace_modify_code)->
> > ftrace_modify_all_code->...->patch_insn_write has no pair of
> > lock&unlock text_mutex, so we shouldn't put assert here.
>
> Hm, are there any chance to get the text_mutex in that path?
> There are ftrace_arch_code_modify_prepare/post_process() in arch/riscv/kernel/ftrace.c,
> aren't those used?
I found ftrace_run_update_code:
 - ftrace_arch_code_modify_prepare()-> mutex_lock(&text_mutex)
 - stop_machine(__ftrace_modify_code, &command, NULL); - >
__ftrace_modify_code -> patch_insn_write -> lockdep_assert_held ->
lockdep_is_held
 - ftrace_arch_code_modify_post_process()-> mutex_unlock(&text_mutex)

Because of stop_machine, __ftrace_modify_code and
ftrace_run_update_code are not the same task. This will cause
lockdep_is_held return 0.

ref to __lock_is_held:
volatile int __lock_is_held(const struct lockdep_map *lock, int read)
{
        struct task_struct *curr = current; // current is from
stop_machine, not ftrace_run_update_code. So never match!
        int i;

        for (i = 0; i < curr->lockdep_depth; i++) {
                struct held_lock *hlock = curr->held_locks + i;

                if (match_held_lock(hlock, lock)) {
                        if (read == -1 || hlock->read == read)
                                return 1;

                        return 0;
                }
        }

        return 0;
}

Does it seem a comm issue? I'm not so expert on lockdep mechanism.

>
> Thank you,
>
> >
> > Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> > ---
> >  arch/riscv/kernel/patch.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> > index 0b55287..2ee5063 100644
> > --- a/arch/riscv/kernel/patch.c
> > +++ b/arch/riscv/kernel/patch.c
> > @@ -55,13 +55,6 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
> >       bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
> >       int ret;
> >
> > -     /*
> > -      * Before reaching here, it was expected to lock the text_mutex
> > -      * already, so we don't need to give another lock here and could
> > -      * ensure that it was safe between each cores.
> > -      */
> > -     lockdep_assert_held(&text_mutex);
> > -
> >       if (across_pages)
> >               patch_map(addr + len, FIX_TEXT_POKE1);
> >
> > --
> > 2.7.4
> >
>
>
> --
> Masami Hiramatsu <mhiramat at kernel.org>



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/