linux-kernel - Re: [PATCH x86/entry: Force rcu_irq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87tuzgxrvz.fsf@nanos.tec.linutronix.de>
Date:   Fri, 12 Jun 2020 16:47:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

Frederic Weisbecker <frederic@...nel.org> writes:

> On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote:
>> The idea of conditionally calling into rcu_irq_enter() only when RCU is
>> not watching turned out to be not completely thought through.
>> 
>> Paul noticed occasional premature end of grace periods in RCU torture
>> testing. Bisection led to the commit which made the invocation of
>> rcu_irq_enter() conditional on !rcu_is_watching().
>> 
>> It turned out that this conditional breaks RCU assumptions about the idle
>> task when the scheduler tick happens to be a nested interrupt. Nested
>> interrupts can happen when the first interrupt invokes softirq processing
>> on return which enables interrupts. If that nested tick interrupt does not
>> invoke rcu_irq_enter() then the nest accounting in RCU claims that this is
>> the first interrupt which might mark a quiescient state and end grace
>> periods prematurely.
>> 
>> Change the condition from !rcu_is_watching() to is_idle_task(current) which
>> enforces that interrupts in the idle task unconditionally invoke
>> rcu_irq_enter() independent of the RCU state.
>> 
>> This is also correct vs. user mode entries in NOHZ full scenarios because
>> user mode entries bring RCU out of EQS and force the RCU irq nesting state
>> accounting to nested. As only the first interrupt can enter from user mode
>> a nested tick interrupt will enter from kernel mode and as the nesting
>> state accounting is forced to nesting it will not do anything stupid even
>> if rcu_irq_enter() has not been invoked.
>> 
>> Fixes: 3eeec3858488 ("x86/entry: Provide idtentry_entry/exit_cond_rcu()")
>> Reported-by: "Paul E. McKenney" <paulmck@...nel.org>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>
> Acked-by: Frederic Weisbecker <frederic@...nel.org>
>
> So, in the end the call to rcu_irq_enter() in irq_enter() is going to
> be useless in x86, right?

x86 is not calling irq_enter() anymore. It's using irq_enter_rcu().