Re: [PATCH] stop_machine: Mark functions as notrace

[Atish Patra]

On Thu, Oct 29, 2020 at 9:06 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> On Thu, Oct 29, 2020 at 10:34 AM Zong Li <zong.li@xxxxxxxxxx> wrote:
> >
> > 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
> No, that is the call trace to show ftrace_make_call ->
> flush_icache_all and we should give notrace on the whole path.
>

Hmm. I am curious to understand how other architectures avoid this problem.
Is it a bigger issue in RISC-V because we have to switch privilege
mode to sync I/D cache ?

> > 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
>
>
>
> --
> Best Regards
>  Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/



-- 
Regards,
Atish