linux-kernel - Re: [PATCH 5/5] random: Defer processing of randomness on PREEMPT

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Date:   Fri, 17 Dec 2021 10:00:52 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Jason@...c4.com, linux-kernel@...r.kernel.org, tytso@....edu,
        tglx@...utronix.de, peterz@...radead.org
Subject: Re: [PATCH 5/5] random: Defer processing of randomness on PREEMPT_RT.

On 2021-12-17 13:23:38 [+1100], Herbert Xu wrote:
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> >
> > Even the IOCTL path must spin with disabled interrupts to avoid dead
> > locks. Therefore it makes no sense if attempt acquire the lock from
> > process or IRQ context. Something like
> >        while (!raw_spin_trylock_irqsave())
> >                cpu_relax()
> 
> What about the TCP socket locking model?
> 
> IOW, the user-space slow path will add itself to a backlog queue
> during contention, and the interrupt fast path will schedule work
> to process any user-space backlog on exit.

I'm sorry, I can't connect the dots here. I was trying to explain that
for the lock in question it is not possible to spin-wait without
disabling interrupts first.

> Cheers,

Sebastian