Re: [PATCH] stop_machine: Mark functions as notrace

[Zong Li]

On Thu, Oct 29, 2020 at 8:23 AM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Oct 28, 2020 at 8:44 AM Guo Ren <guoren@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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
> >

It seems to me that the problem happens on the waiting threads, it
doesn't cause the issue on the patching code thread, so it is OK that
these functions are traceable. I probably don't figure out all
possible situations, do you find any issue and reason to change the
annotation of these functions?

> > On Wed, Oct 21, 2020 at 3:38 PM Zong Li <zong.li@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >  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