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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [patch V2 11/15] completion: Use simple wait queues
In-Reply-To: <20200319003351.GA211584@google.com>
References: <20200318204302.693307984@linutronix.de> <20200318204408.521507446@linutronix.de> <20200319003351.GA211584@google.com>
Date:   Thu, 19 Mar 2020 01:44:26 +0100
Message-ID: <87a74ddvh1.fsf@nanos.tec.linutronix.de>
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Joel,

Joel Fernandes <joel@joelfernandes.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