linux-kernel - Re: [RFC 0/1] wait queue head; locking considerations

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Date:	Fri, 06 Dec 2013 20:34:04 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 0/1] wait queue head; locking considerations

Hi,

06.12.2013, 19:24, "Paul Gortmaker" <paul.gortmaker@...driver.com>:
> Peter pointed out[1] that the simple wait queues in RT could
> run any number of wakeups with the wait queue head lock held.
>
> It turns out that this isnt really specific to RT, but it is
> mainline too -- all the __wake_up_XYZ that in turn use the
> __wake_up_common boiler plate end up looping over all the
> callbacks (normally ttwu) while holding the head lock.
<snip>

One more problem with wait queues in RT case.

Wait queues moving from spinlocks to rt_mutexes makes timerfd
not irqsafe on RT. This makes it unusable for some user tasks.

>  fs/timerfd.c                                       | 26 ++++++-------
>  include/linux/wait.h                               | 36 +++++++++++++++---
>  kernel/sched/completion.c                          | 24 ++++++------
>  kernel/sched/core.c                                |  8 ++--
>  kernel/sched/wait.c                                | 44 +++++++++++-----------
>  mm/filemap.c                                       |  4 +-
>  net/sunrpc/sched.c                                 |  4 +-
>  14 files changed, 150 insertions(+), 126 deletions(-)
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