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

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Message-ID: <9bb6db07-12c4-c272-3a98-baed2ac2f738@deltatee.com>
Date:   Wed, 18 Mar 2020 16:28:34 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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



On 2020-03-18 2:43 p.m., Thomas Gleixner wrote:
> There is no semantical or functional change:
> 
>   - completions use the exclusive wait mode which is what swait provides
> 
>   - complete() wakes one exclusive waiter
> 
>   - complete_all() wakes all waiters while holding the lock which protects
>     the wait queue against newly incoming waiters. The conversion to swait
>     preserves this behaviour.
> 
> complete_all() might cause unbound latencies with a large number of waiters
> being woken at once, but most complete_all() usage sites are either in
> testing or initialization code or have only a really small number of
> concurrent waiters which for now does not cause a latency problem. Keep it
> simple for now.

Seems like it would be worth adding a note for this to the
complete_all() doc string. Otherwise developers will not likely find out
about this issue and may not keep it as simple as you'd like.

Logan