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Date: Wed, 22 Jan 2014 14:47:42 -0500
From: Paul Gortmaker <paul.gortmaker@...driver.com>
To: Steven Rostedt <rostedt@...dmis.org>,
LKML <linux-kernel@...r.kernel.org>,
linux-rt-users <linux-rt-users@...r.kernel.org>
CC: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH] preempt: Debug for possible missed preemption checks
On 14-01-16 11:57 PM, Steven Rostedt wrote:
> Peter Zijlstra mentioned that he wanted to catch the following problem:
>
> local_irq_disable();
> preempt_disable();
> local_irq_enable();
>
> local_irq_disable();
> preempt_enable();
> local_irq_enable();
>
[...]
> +
> +static void update_pied_state(enum pied_stat_type type, bool enable,
> + unsigned long ip)
> +{
> + struct stack_trace *trace;
> + int state, new_state;
> + unsigned long flags;
> +
> + if (pied_failed)
> + return;
> +
> + if (this_cpu_read(pied_recursive))
> + return;
Maybe I'm just missing something obvious, but it wasn't clear to me
how or what the recursive check above would reliably protect against,
when the increment is ~20 lines below and not a cmpxchg here.
Thanks,
Paul.
--
> +
> + /*
> + * Boot up may start with interrupts and/or preemption
> + * disabled. We can't start the state updates till
> + * we have synced with the initial state.
> + */
> + if (!this_cpu_read(pied_initialized)) {
> + /*
> + * The first time we enable preemption with interrupts
> + * enabled on a CPU, start the state transactions.
> + */
> + if (!in_interrupt() && type == PIED_STATE_PREEMPT &&
> + enable && !irqs_disabled())
> + this_cpu_write(pied_initialized, 1);
> + return;
> + }
> +
> + if (type == PIED_STATE_INTERRUPT) {
> + if (enable == false) {
> + /* Ignore nested disabling of interrupts */
> + if (this_cpu_read(pied_irqsoff))
> + return;
> + this_cpu_write(pied_irqsoff, 1);
> + } else
> + this_cpu_write(pied_irqsoff, 0);
> + }
> +
> + this_cpu_inc(pied_recursive);
> + raw_local_irq_save(flags);
> +
> + state = this_cpu_read(current_pied_state);
> +
> + switch (type) {
> + case PIED_STATE_PREEMPT:
> + new_state = pied_state_trans[state].preempt_change;
> + switch (new_state) {
> + case 0: case 1: case 6: case 7:
> + if (!enable)
> + pied_state_bug(type, enable, state, new_state);
> + break;
> + default:
> + if (enable)
> + pied_state_bug(type, enable, state, new_state);
> + break;
> + }
> + break;
> + case PIED_STATE_INTERRUPT:
> + new_state = pied_state_trans[state].interrupt_change;
> + switch (new_state) {
> + case 0: case 2: case 3: case 7:
> + if (!enable)
> + pied_state_bug(type, enable, state, new_state);
> + break;
> + default:
> + if (enable)
> + pied_state_bug(type, enable, state, new_state);
> + break;
> + }
> + break;
> + }
> +
> + switch (new_state) {
> + case PIED_DANGEROUS_STATE:
> + /*
> + * If we are in an interrupt, then we need to switch
> + * to state 3 to prevent from going into state 5, 6 and 7.
> + *
> + * PDIEX ==> PDIE
> + */
> + if (in_interrupt()) {
> + new_state = 3;
> + break;
> + }
> + trace = this_cpu_ptr(&pied_stack_trace);
> + trace->nr_entries = 0;
> + trace->max_entries = PIED_STACK_MAX;
> + trace->entries = this_cpu_ptr(pied_stack);
> +
> + trace->skip = 3;
> +
> + save_stack_trace(trace);
> +
> + break;
> + case PIED_BAD_STATE:
> +
> + /*
> + * Interrupts themselves do not cause problems as they
> + * always check NEED_RESCHED when going back to normal context.
> + *
> + * PEIEX ==> PEIE
> + */
> + if (in_interrupt()) {
> + new_state = 0;
> + break;
> + }
> +
> + lockdep_off();
> + pied_failed = true;
> + printk("\n");
> + printk("===============================\n");
> + printk("[INFO: preempt check hit problem state]\n");
> + print_irqtrace_events(current);
> + printk("Entered dangerous state at: \n");
> + print_stack_trace(this_cpu_ptr(&pied_stack_trace), 2);
> + printk("\nstack backtrace:\n");
> + dump_stack();
> + print_pied_trail();
> + lockdep_on();
> + break;
> + }
> + this_cpu_write(current_pied_state, new_state);
> + update_pied_trail(new_state, ip, irqs_disabled_flags(flags));
> + raw_local_irq_restore(flags);
> + this_cpu_dec(pied_recursive);
> +}
> +
> +void trace_preempt_on(unsigned long a0, unsigned long a1)
> +{
> + time_preempt_on(a0, a1);
> + update_pied_state(PIED_STATE_PREEMPT, true, a0);
> +}
> +
> +void trace_preempt_off(unsigned long a0, unsigned long a1)
> +{
> + time_preempt_off(a0, a1);
> + update_pied_state(PIED_STATE_PREEMPT, false, a0);
> +}
> +#endif /* CONFIG_PREEMPT */
> Index: linux-trace.git/kernel/sched/core.c
> ===================================================================
> --- linux-trace.git.orig/kernel/sched/core.c
> +++ linux-trace.git/kernel/sched/core.c
> @@ -2414,7 +2414,8 @@ void __kprobes preempt_count_add(int val
> DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >=
> PREEMPT_MASK - 10);
> #endif
> - if (preempt_count() == val)
> + /* PREEMPT_ACTIVE gets set directly, it must be ignored */
> + if ((preempt_count() & ~PREEMPT_ACTIVE) == val)
> trace_preempt_off(ip, parent_ip);
> }
> EXPORT_SYMBOL(preempt_count_add);
> @@ -2435,7 +2436,8 @@ void __kprobes preempt_count_sub(int val
> return;
> #endif
>
> - if (preempt_count() == val)
> + /* PREEMPT_ACTIVE gets set directly, it must be ignored */
> + if ((preempt_count() & ~PREEMPT_ACTIVE) == val)
> trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> __preempt_count_sub(val);
> }
> Index: linux-trace.git/kernel/softirq.c
> ===================================================================
> --- linux-trace.git.orig/kernel/softirq.c
> +++ linux-trace.git/kernel/softirq.c
> @@ -111,7 +111,8 @@ static void __local_bh_disable(unsigned
> trace_softirqs_off(ip);
> raw_local_irq_restore(flags);
>
> - if (preempt_count() == cnt)
> + /* PREEMPT_ACTIVE gets set directly, it must be ignored */
> + if ((preempt_count() & ~PREEMPT_ACTIVE) == cnt)
> trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> }
> #else /* !CONFIG_TRACE_IRQFLAGS */
> Index: linux-trace.git/kernel/trace/trace_irqsoff.c
> ===================================================================
> --- linux-trace.git.orig/kernel/trace/trace_irqsoff.c
> +++ linux-trace.git/kernel/trace/trace_irqsoff.c
> @@ -516,13 +516,13 @@ EXPORT_SYMBOL(trace_hardirqs_off_caller)
> #endif /* CONFIG_IRQSOFF_TRACER */
>
> #ifdef CONFIG_PREEMPT_TRACER
> -void trace_preempt_on(unsigned long a0, unsigned long a1)
> +void time_preempt_on(unsigned long a0, unsigned long a1)
> {
> if (preempt_trace() && !irq_trace())
> stop_critical_timing(a0, a1);
> }
>
> -void trace_preempt_off(unsigned long a0, unsigned long a1)
> +void time_preempt_off(unsigned long a0, unsigned long a1)
> {
> if (preempt_trace() && !irq_trace())
> start_critical_timing(a0, a1);
>
> --
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