netdev - Re: [patch V2 11/15] completion: Use simple wait queues

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Message-ID: <87a74ddvh1.fsf@nanos.tec.linutronix.de>
Date:   Thu, 19 Mar 2020 01:44:26 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Logan Gunthorpe <logang@...tatee.com>,
        Kurt Schwemmer <kurt.schwemmer@...rosemi.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [patch V2 11/15] completion: Use simple wait queues

Joel,

Joel Fernandes <joel@...lfernandes.org> writes:
> On Wed, Mar 18, 2020 at 09:43:13PM +0100, Thomas Gleixner wrote:
>> The spinlock in the wait queue head cannot be replaced by a raw_spinlock
>> because:
>> 
>>   - wait queues can have custom wakeup callbacks, which acquire other
>>     spinlock_t locks and have potentially long execution times
>
> Cool, makes sense.
>
>>   - wake_up() walks an unbounded number of list entries during the wake up
>>     and may wake an unbounded number of waiters.
>
> Just to clarify here, wake_up() will really wake up just 1 waiter if all the
> waiters on the queue are exclusive right? So in such scenario at least, the
> "unbounded number of waiters" would not be an issue if everything waiting was
> exclusive and waitqueue with wake_up() was used. Please correct me if I'm
> wrong about that though.

Correct.

> So the main reasons to avoid waitqueue in favor of swait (as you mentioned)
> would be the sleep-while-atomic issue in truly atomic context on RT, and the
> fact that callbacks can take a long time.

Yes.

Thanks,

        tglx