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Date: Wed, 26 Sep 2012 09:59:43 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Sasha Levin <levinsasha928@...il.com>,
Michael Wang <wangyun@...ux.vnet.ibm.com>,
Dave Jones <davej@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RCU idle CPU detection is broken in linux-next
On Wed, Sep 26, 2012 at 05:46:13PM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 25, 2012 at 11:36:54AM -0700, Paul E. McKenney wrote:
> > On Tue, Sep 25, 2012 at 08:28:23PM +0200, Sasha Levin wrote:
> > > On 09/25/2012 02:06 PM, Frederic Weisbecker wrote:
> > > > Sasha, sorry to burden you with more testing request.
> > > > Could you please try out this new branch? It includes some fixes after Wu Fenguang and
> > > > Dan Carpenter reports (not related to your warnings though) and a patch on the top
> > > > of the pile to ensure I diagnosed well the problem, which return immediately from
> > > > rcu_user_*() APIs if we are in an interrupt.
> > > >
> > > > This way we'll have a clearer view. I also would like to know if there are other
> > > > problems with the rcu user mode.
> > > >
> > > > Thanks!
> > >
> > > Alrighty, I don't see any warnings anymore.
> > >
> > > I'll keep everything running just in case.
> >
> > Very good news!!! Thank you both!!!
>
> So, I've pushed the fixes in a new branch.
>
> Changes are:
>
> * Added in_interrupt() checks in "rcu: New rcu_user_enter() and rcu_user_exit() APIs"
> so that rcu_user_enter/exit() don't nest in rcu_irq_enter/exit().
>
> We'll need a longer term solution I guess because of:
> - Irq bad nesting
> - An exception could happen in the middle of irq_exit(), although that's quite
> unlikely (ie: between sub_preempt_count(HARDIRQ) and add_preempt_count(SOFTIRQ) or between
> sub_preempt_count(SOFTIRQ or HARDIRQ) and rcu_irq_exit().
> May be I should rather check for (rdtp->dynticks_nesting & (DYNTICK_TASK_FLAG -1))
> to find out if we are in the middle of an irq from an RCU POV.
>
>
> * Fix the !notifie_die(...) == NOTIFY_STOP to become notifie_die(...) != NOTIFY_STOP.
> Reported by Wu Fenguang and Dan Carpenter. Fixes are folded inside:
> "x86: Exception hooks for userspace RCU extended QS".
>
> The branch (rebase on top of your rcu/idle) is:
>
> git://github.com/fweisbec/linux-dynticks.git
> rcu/idle-for-v3.7-take5
Thank you, Frederic! I have pushed this to rcu/idle in -rcu, and am
testing in parallel.
Thanx, Paul
> Diff against your rcu/idle:
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index cb20776..3789675 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -232,8 +232,8 @@ DO_ERROR_INFO(X86_TRAP_AC, SIGBUS, "alignment check", alignment_check,
> dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
> {
> exception_enter(regs);
> - if (!notify_die(DIE_TRAP, "stack segment", regs, error_code,
> - X86_TRAP_SS, SIGBUS) == NOTIFY_STOP) {
> + 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);
> @@ -285,8 +285,8 @@ do_general_protection(struct pt_regs *regs, long error_code)
>
> tsk->thread.error_code = error_code;
> tsk->thread.trap_nr = X86_TRAP_GP;
> - if (!notify_die(DIE_GPF, "general protection fault", regs, error_code,
> - X86_TRAP_GP, SIGSEGV) == NOTIFY_STOP)
> + if (notify_die(DIE_GPF, "general protection fault", regs, error_code,
> + X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
> die("general protection fault", regs, error_code);
> goto exit;
> }
> @@ -678,8 +678,8 @@ dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code)
> info.si_errno = 0;
> info.si_code = ILL_BADSTK;
> info.si_addr = NULL;
> - if (!notify_die(DIE_TRAP, "iret exception", regs, error_code,
> - X86_TRAP_IRET, SIGILL) == NOTIFY_STOP) {
> + if (notify_die(DIE_TRAP, "iret exception", regs, error_code,
> + X86_TRAP_IRET, SIGILL) != NOTIFY_STOP) {
> do_trap(X86_TRAP_IRET, SIGILL, "iret exception", regs, error_code,
> &info);
> }
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 72453cf..4fb2376 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -418,6 +418,17 @@ void rcu_user_enter(void)
> unsigned long flags;
> struct rcu_dynticks *rdtp;
>
> + /*
> + * Some contexts may involve an exception occuring in an irq,
> + * leading to that nesting:
> + * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
> + * This would mess up the dyntick_nesting count though. And rcu_irq_*()
> + * helpers are enough to protect RCU uses inside the exception. So
> + * just return immediately if we detect we are in an IRQ.
> + */
> + if (in_interrupt())
> + return;
> +
> WARN_ON_ONCE(!current->mm);
>
> local_irq_save(flags);
> @@ -566,6 +577,17 @@ void rcu_user_exit(void)
> unsigned long flags;
> struct rcu_dynticks *rdtp;
>
> + /*
> + * Some contexts may involve an exception occuring in an irq,
> + * leading to that nesting:
> + * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
> + * This would mess up the dyntick_nesting count though. And rcu_irq_*()
> + * helpers are enough to protect RCU uses inside the exception. So
> + * just return immediately if we detect we are in an IRQ.
> + */
> + if (in_interrupt())
> + return;
> +
> local_irq_save(flags);
> rdtp = &__get_cpu_var(rcu_dynticks);
> if (rdtp->in_user) {
>
--
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