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

[Alexandre Chartre]

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@xxxxxxxxxxxxx>
> ---
>   arch/x86/entry/common.c         |  119 ++++++++++++++++++++++++++++++++++------
>   arch/x86/include/asm/idtentry.h |    3 +
>   2 files changed, 106 insertions(+), 16 deletions(-)