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  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]
Date:   Fri, 12 Jun 2020 16:26:22 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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

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?

Powered by blists - more mailing lists