Re: [patch V4 part 5 07/31] x86/entry: Provide idtentry_entry/exit_cond_rcu()

Alexandre Chartre
2020-05-11T13:55:24Z

On 5/5/20 3:53 PM, Thomas Gleixner wrote:
> The pagefault handler cannot use the regular idtentry_enter() because on
> that invokes rcu_irq_enter() the pagefault was caused in the kernel.

I am struggling to understand this part of the sentence: "because on
that invokes rcu_irq_enter() the pagefault was caused in the kernel."

Do you mean: "because that invokes rcu_irq_enter() if the pagefault was
caused in the kernel." ?

alex.

> Not a
> problem per se, but kernel side page faults can schedule which is not
> possible without invoking rcu_irq_exit().
>
> Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual
> pagefault handling code is possible, but not pretty either.
>
> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only
> when RCU is not watching. While this is not a legit kernel #PF establishing
> RCU before handling it avoids RCU side effects which might affect
> debugability.
>
> The function is also useful for implementing lightweight scheduler IPI
> entry handling later.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/x86/entry/common.c | 119 ++++++++++++++++++++++++++++++++++------
> arch/x86/include/asm/idtentry.h | 3 +
> 2 files changed, 106 insertions(+), 16 deletions(-)