Re: [PATCH] stop_machine: Mark functions as notrace

[Atish Patra <atishp@atishpatra.org>]

On Wed, Oct 28, 2020 at 8:44 AM Guo Ren <guoren@kernel.org> wrote:
>
> Hi Zong & Atish,
>
> In our 2 harts c910 chip, we found:
>
> echo function > /sys/kernel/debug/tracing/current_tracer
> echo function_graph > /sys/kernel/debug/tracing/current_tracer
> echo function > /sys/kernel/debug/tracing/current_tracer
> echo function_graph > /sys/kernel/debug/tracing/current_tracer
>
> Then one core halted at stop_machine_yield:
> arch_cpu_idle () at arch/riscv/kernel/process.c:39
> 39              local_irq_enable();
> (gdb) i th
>   Id   Target Id         Frame
> * 1    Thread 1 (CPU#0)  arch_cpu_idle () at arch/riscv/kernel/process.c:39
>   2    Thread 2 (CPU#1)  stop_machine_yield
> (cpumask=0xffffffe001371fa8 <__cpu_online_mask>) at
> ./arch/riscv/include/asm/vdso/processor.h:12
> (gdb) thread 2
> [Switching to thread 2 (Thread 2)]
> #0  stop_machine_yield (cpumask=0xffffffe001371fa8
> <__cpu_online_mask>) at ./arch/riscv/include/asm/vdso/processor.h:12
> 12              __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
>
> With your patch, it's solved. For this patch, I'll give:
> Tested by: Guo Ren <guoren@kernel.org>
>
> But that's not enough, we still need:
>
> diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> index 226ccce..12b8808 100644
> --- a/arch/riscv/kernel/sbi.c
> +++ b/arch/riscv/kernel/sbi.c
> @@ -376,7 +376,7 @@ EXPORT_SYMBOL(sbi_send_ipi);
>   *
>   * Return: None
>   */
> -void sbi_remote_fence_i(const unsigned long *hart_mask)
> +void notrace sbi_remote_fence_i(const unsigned long *hart_mask)
>  {
>         __sbi_rfence(SBI_EXT_RFENCE_REMOTE_FENCE_I,
>                      hart_mask, 0, 0, 0, 0);
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 400b945d..9467d987 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -9,12 +9,12 @@
>
>  #include <asm/sbi.h>
>
> -static void ipi_remote_fence_i(void *info)
> +static void notrace ipi_remote_fence_i(void *info)
>  {
>         return local_flush_icache_all();
>  }
>
> -void flush_icache_all(void)
> +void notrace flush_icache_all(void)
>  {
>         if (IS_ENABLED(CONFIG_RISCV_SBI))
>                 sbi_remote_fence_i(NULL);
>

Did you see any issue if these functions are not marked as notrace ?

As per Zong's explanation, the issue was that the other harts already
fetched the next 2 nops and
executed 1 while kernel patching replaced other with one of the auipc
+ jalr pair.

@Zong can correct me if I am wrong.

These functions are too far ahead. Can it cause such issues ? If yes,
then we need to mark each and every function
that can be invoked from patch_text_nosync and are not inlined.

That includes copy_to_kernel_nofault, __sbi_rfence_v02,
__sbi_rfence_v02_call, sbi_ecall.

Few of these functions may be inlined by compiler. Can we depend on that ?

> Because:
> (gdb) bt
> #0  flush_icache_all () at arch/riscv/mm/cacheflush.c:20
> #1  0xffffffe00020473a in patch_text_nosync (addr=<optimized out>, insns=
>     <optimized out>, len=<optimized out>) at arch/riscv/kernel/patch.c:96
> #2  0xffffffe000206792 in ftrace_make_call (rec=<optimized out>,
> addr=<optimized out>) at arch/riscv/kernel/ftrace.c:109
> #3  0xffffffe0002c9be4 in __ftrace_replace_code (rec=0xffffffe01ae40020, e
>     nable=true) at kernel/trace/ftrace.c:2503
> #4  0xffffffe0002ca092 in ftrace_replace_code (mod_flags=<optimized
>     out>) at kernel/trace/ftrace.c:2530
> #5  0xffffffe0002ca24a in ftrace_modify_all_code (command=9) at kernel
>    /trace/ftrace.c:2677
> #6  0xffffffe0002ca2ee in __ftrace_modify_code (data=<optimized out>) at
>    kernel/trace/ftrace.c:2703
> #7  0xffffffe0002c1390 in multi_cpu_stop (data=0x0) at kernel/stop_machin
>    e.c:224
> #8  0xffffffe0002c0fbe in cpu_stopper_thread (cpu=<optimized out>) at kern
>    el/stop_machine.c:491
> #9  0xffffffe0002343be in smpboot_thread_fn (data=0x0) at kernel/smpboot.
>    c:165
> #10 0xffffffe00022f894 in kthread (_create=0xffffffe01af13040) at kern
>    el/kthread.c:292
> #11 0xffffffe000201fac in handle_exception () at arch/riscv/kernel/entry.S:236
>
> On Wed, Oct 21, 2020 at 3:38 PM Zong Li <zong.li@sifive.com> wrote:
> >
> > Like the commit cb9d7fd51d9f ("watchdog: Mark watchdog touch functions
> > as notrace"), some architectures assume that the stopped CPUs don't make
> > function calls to traceable functions when they are in the stopped
> > state. For example, it causes unexpected kernel crashed when switching
> > tracer on RISC-V.
> >
> > The following patches added calls to these two functions, fix it by
> > adding the notrace annotations.
> >
> > Fixes: 4ecf0a43e729 ("processor: get rid of cpu_relax_yield")
> > Fixes: 366237e7b083 ("stop_machine: Provide RCU quiescent state in
> > multi_cpu_stop()")
> >
> > Signed-off-by: Zong Li <zong.li@sifive.com>
> > ---
> >  kernel/rcu/tree.c     | 2 +-
> >  kernel/stop_machine.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 06895ef85d69..2a52f42f64b6 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -409,7 +409,7 @@ bool rcu_eqs_special_set(int cpu)
> >   *
> >   * The caller must have disabled interrupts and must not be idle.
> >   */
> > -void rcu_momentary_dyntick_idle(void)
> > +notrace void rcu_momentary_dyntick_idle(void)
> >  {
> >         int special;
> >
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index 865bb0228ab6..890b79cf0e7c 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -178,7 +178,7 @@ static void ack_state(struct multi_stop_data *msdata)
> >                 set_state(msdata, msdata->state + 1);
> >  }
> >
> > -void __weak stop_machine_yield(const struct cpumask *cpumask)
> > +notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
> >  {
> >         cpu_relax();
> >  }
> > --
> > 2.28.0
> >
>
>
> --
> Best Regards
>  Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/



-- 
Regards,
Atish

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