Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() | Date | Tue, 19 May 2020 22:20:02 +0200 |
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Thomas Gleixner <tglx@linutronix.de> writes: > Andy Lutomirski <luto@kernel.org> writes: >> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote: >>> The pagefault handler cannot use the regular idtentry_enter() because that >>> invokes rcu_irq_enter() if the pagefault was caused in the kernel. 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 would be possible, but not pretty either. >>> >>> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only >>> when RCU is not watching. The conditional RCU enabling is a correctness >>> issue: A kernel page fault which hits a RCU idle reason can neither >>> schedule nor is it likely to survive. But avoiding RCU warnings or RCU side >>> effects is at least increasing the chance for useful debug output. >>> >>> The function is also useful for implementing lightweight reschedule IPI and >>> KVM posted interrupt IPI entry handling later. >> >> Why is this conditional? That is, couldn't we do this for all >> idtentry_enter() calls instead of just for page faults? Evil things >> like NMI shouldn't go through this path at all. > > I thought about that, but then ended up with the conclusion that RCU > might be unhappy, but my conclusion might be fundamentally wrong.
It's about this:
rcu_nmi_enter() { if (!rcu_is_watching()) { make it watch; } else if (!in_nmi()) { do_magic_nohz_dyntick_muck(); }
So if we do all irq/system vector entries conditional then the do_magic() gets never executed. After that I got lost...
Thanks,
tglx
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