Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

Paul E. McKenney
2014-11-21T22:07:13Z

On Fri, Nov 21, 2014 at 01:32:50PM -0800, Andy Lutomirski wrote:
> On Fri, Nov 21, 2014 at 1:26 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > We currently pretend that IST context is like standard exception
> > context, but this is incorrect. IST entries from userspace are like
> > standard exceptions except that they use per-cpu stacks, so they are
> > atomic. IST entries from kernel space are like NMIs from RCU's
> > perspective -- they are not quiescent states even if they
> > interrupted the kernel during a quiescent state.
> >
> > Add and use ist_enter and ist_exit to track IST context. Even
> > though x86_32 has no IST stacks, we track these interrupts the same
> > way.
>
> I should add:
>
> I have no idea why RCU read-side critical sections are safe inside
> __do_page_fault today. It's guarded by exception_enter(), but that
> doesn't do anything if context tracking is off, and context tracking
> is usually off. What am I missing here?

Ah! There are three cases:

1. Context tracking is off on a non-idle CPU. In this case, RCU is
still paying attention to CPUs running in both userspace and in
the kernel. So if a page fault happens, RCU will be set up to
notice any RCU read-side critical sections.

2. Context tracking is on on a non-idle CPU. In this case, RCU
might well be ignoring userspace execution: NO_HZ_FULL and
all that. However, as you pointed out, in this case the
context-tracking code lets RCU know that we have entered the
kernel, which means that RCU will again be paying attention to
RCU read-side critical sections.

3. The CPU is idle. In this case, RCU is ignoring the CPU, so
if we take a page fault when context tracking is off, life
will be hard. But the kernel is not supposed to take page
faults in the idle loop, so this is not a problem.

Make sense?

Thanx, Paul

> --Andy
>
> >
> > This fixes two issues:
> >
> > - Scheduling from an IST interrupt handler will now warn. It would
> > previously appear to work as long as we got lucky and nothing
> > overwrote the stack frame. (I don't know of any bugs in this
> > that would trigger the warning, but it's good to be on the safe
> > side.)
> >
> > - RCU handling in IST context was dangerous. As far as I know,
> > only machine checks were likely to trigger this, but it's good to
> > be on the safe side.
> >
> > Note that the machine check handlers appears to have been missing
> > any context tracking at all before this patch.
> >
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Frédéric Weisbecker <fweisbec@gmail.com>
> > Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> > ---
> > arch/x86/include/asm/traps.h | 4 +++
> > arch/x86/kernel/cpu/mcheck/mce.c | 5 ++++
> > arch/x86/kernel/cpu/mcheck/p5.c | 6 +++++
> > arch/x86/kernel/cpu/mcheck/winchip.c | 5 ++++
> > arch/x86/kernel/traps.c | 49 ++++++++++++++++++++++++++++++------
> > 5 files changed, 61 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> > index bc8352e7010a..eb16a61bfd06 100644
> > --- a/arch/x86/include/asm/traps.h
> > +++ b/arch/x86/include/asm/traps.h
> > @@ -1,6 +1,7 @@
> > #ifndef _ASM_X86_TRAPS_H
> > #define _ASM_X86_TRAPS_H
> >
> > +#include <linux/context_tracking_state.h>
> > #include <linux/kprobes.h>
> >
> > #include <asm/debugreg.h>
> > @@ -109,6 +110,9 @@ asmlinkage void smp_thermal_interrupt(void);
> > asmlinkage void mce_threshold_interrupt(void);
> > #endif
> >
> > +extern enum ctx_state ist_enter(struct pt_regs *regs);
> > +extern void ist_exit(struct pt_regs *regs, enum ctx_state prev_state);
> > +
> > /* Interrupts/Exceptions */
> > enum {
> > X86_TRAP_DE = 0, /* 0, Divide-by-zero */
> > diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> > index 61a9668cebfd..b72509d77337 100644
> > --- a/arch/x86/kernel/cpu/mcheck/mce.c
> > +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> > @@ -43,6 +43,7 @@
> > #include <linux/export.h>
> >
> > #include <asm/processor.h>
> > +#include <asm/traps.h>
> > #include <asm/mce.h>
> > #include <asm/msr.h>
> >
> > @@ -1016,6 +1017,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
> > {
> > struct mca_config *cfg = &mca_cfg;
> > struct mce m, *final;
> > + enum ctx_state prev_state;
> > int i;
> > int worst = 0;
> > int severity;
> > @@ -1038,6 +1040,8 @@ void do_machine_check(struct pt_regs *regs, long error_code)
> > DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
> > char *msg = "Unknown";
> >
> > + prev_state = ist_enter(regs);
> > +
> > this_cpu_inc(mce_exception_count);
> >
> > if (!cfg->banks)
> > @@ -1168,6 +1172,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
> > mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
> > out:
> > sync_core();
> > + ist_exit(regs, prev_state);
> > }
> > EXPORT_SYMBOL_GPL(do_machine_check);
> >
> > diff --git a/arch/x86/kernel/cpu/mcheck/p5.c b/arch/x86/kernel/cpu/mcheck/p5.c
> > index a3042989398c..ec2663a708e4 100644
> > --- a/arch/x86/kernel/cpu/mcheck/p5.c
> > +++ b/arch/x86/kernel/cpu/mcheck/p5.c
> > @@ -8,6 +8,7 @@
> > #include <linux/smp.h>
> >
> > #include <asm/processor.h>
> > +#include <asm/traps.h>
> > #include <asm/mce.h>
> > #include <asm/msr.h>
> >
> > @@ -17,8 +18,11 @@ int mce_p5_enabled __read_mostly;
> > /* Machine check handler for Pentium class Intel CPUs: */
> > static void pentium_machine_check(struct pt_regs *regs, long error_code)
> > {
> > + enum ctx_state prev_state;
> > u32 loaddr, hi, lotype;
> >
> > + prev_state = ist_enter(regs);
> > +
> > rdmsr(MSR_IA32_P5_MC_ADDR, loaddr, hi);
> > rdmsr(MSR_IA32_P5_MC_TYPE, lotype, hi);
> >
> > @@ -33,6 +37,8 @@ static void pentium_machine_check(struct pt_regs *regs, long error_code)
> > }
> >
> > add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
> > +
> > + ist_exit(regs, prev_state);
> > }
> >
> > /* Set up machine check reporting for processors with Intel style MCE: */
> > diff --git a/arch/x86/kernel/cpu/mcheck/winchip.c b/arch/x86/kernel/cpu/mcheck/winchip.c
> > index 7dc5564d0cdf..bd5d46a32210 100644
> > --- a/arch/x86/kernel/cpu/mcheck/winchip.c
> > +++ b/arch/x86/kernel/cpu/mcheck/winchip.c
> > @@ -7,14 +7,19 @@
> > #include <linux/types.h>
> >
> > #include <asm/processor.h>
> > +#include <asm/traps.h>
> > #include <asm/mce.h>
> > #include <asm/msr.h>
> >
> > /* Machine check handler for WinChip C6: */
> > static void winchip_machine_check(struct pt_regs *regs, long error_code)
> > {
> > + enum ctx_state prev_state = ist_enter(regs);
> > +
> > printk(KERN_EMERG "CPU0: Machine Check Exception.\n");
> > add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
> > +
> > + ist_exit(regs, prev_state);
> > }
> >
> > /* Set up machine check reporting on the Winchip C6 series */
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index 0d0e922fafc1..f5c4b8813774 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -107,6 +107,39 @@ static inline void preempt_conditional_cli(struct pt_regs *regs)
> > preempt_count_dec();
> > }
> >
> > +enum ctx_state ist_enter(struct pt_regs *regs)
> > +{
> > + /*
> > + * We are atomic because we're on the IST stack (or we're on x86_32,
> > + * in which case we still shouldn't schedule.
> > + */
> > + preempt_count_add(HARDIRQ_OFFSET);
> > +
> > + if (user_mode_vm(regs)) {
> > + /* Other than that, we're just an exception. */
> > + return exception_enter();
> > + } else {
> > + /*
> > + * We might have interrupted pretty much anything. In
> > + * fact, if we're a machine check, we can even interrupt
> > + * NMI processing. We don't want in_nmi() to return true,
> > + * but we need to notify RCU.
> > + */
> > + rcu_nmi_enter();
> > + return IN_KERNEL; /* the value is irrelevant. */
> > + }
> > +}
> > +
> > +void ist_exit(struct pt_regs *regs, enum ctx_state prev_state)
> > +{
> > + preempt_count_sub(HARDIRQ_OFFSET);
> > +
> > + if (user_mode_vm(regs))
> > + return exception_exit(prev_state);
> > + else
> > + rcu_nmi_exit();
> > +}
> > +
> > static nokprobe_inline int
> > do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
> > struct pt_regs *regs, long error_code)
> > @@ -244,14 +277,14 @@ dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
> > {
> > enum ctx_state prev_state;
> >
> > - prev_state = exception_enter();
> > + prev_state = ist_enter(regs);
> > if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
> > X86_TRAP_SS, SIGBUS) != NOTIFY_STOP) {
> > preempt_conditional_sti(regs);
> > do_trap(X86_TRAP_SS, SIGBUS, "stack segment", regs, error_code, NULL);
> > preempt_conditional_cli(regs);
> > }
> > - exception_exit(prev_state);
> > + ist_exit(regs, prev_state);
> > }
> >
> > dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> > @@ -259,8 +292,8 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> > static const char str[] = "double fault";
> > struct task_struct *tsk = current;
> >
> > - exception_enter();
> > - /* Return not checked because double check cannot be ignored */
> > + ist_enter(regs);
> > + /* Return not checked because double fault cannot be ignored */
> > notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);
> >
> > tsk->thread.error_code = error_code;
> > @@ -343,7 +376,7 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
> > if (poke_int3_handler(regs))
> > return;
> >
> > - prev_state = exception_enter();
> > + prev_state = ist_enter(regs);
> > #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
> > if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
> > SIGTRAP) == NOTIFY_STOP)
> > @@ -369,7 +402,7 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
> > preempt_conditional_cli(regs);
> > debug_stack_usage_dec();
> > exit:
> > - exception_exit(prev_state);
> > + ist_exit(regs, prev_state);
> > }
> > NOKPROBE_SYMBOL(do_int3);
> >
> > @@ -433,7 +466,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
> > unsigned long dr6;
> > int si_code;
> >
> > - prev_state = exception_enter();
> > + prev_state = ist_enter(regs);
> >
> > get_debugreg(dr6, 6);
> >
> > @@ -508,7 +541,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
> > debug_stack_usage_dec();
> >
> > exit:
> > - exception_exit(prev_state);
> > + ist_exit(regs, prev_state);
> > }
> > NOKPROBE_SYMBOL(do_debug);
> >
> > --
> > 1.9.3
> >
>
>
>
> --
> Andy Lutomirski
> AMA Capital Management, LLC
>