linux-kernel - Re: [PATCH v4 3/4] futex: Document ordering guarantees

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Message-ID: <52B35C64.3050504@infradead.org>
Date:	Thu, 19 Dec 2013 12:51:48 -0800
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>, linux-kernel@...r.kernel.org
CC:	mingo@...hat.com, dvhart@...ux.intel.com, peterz@...radead.org,
	tglx@...utronix.de, paulmck@...ux.vnet.ibm.com, efault@....de,
	jeffm@...e.com, torvalds@...ux-foundation.org, jason.low2@...com,
	Waiman.Long@...com, tom.vaden@...com, scott.norton@...com,
	aswin@...com, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 3/4] futex: Document ordering guarantees

On 12/19/13 12:45, Davidlohr Bueso wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> That's essential, if you want to hack on futexes.
> 
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Darren Hart <dvhart@...ux.intel.com>
> Acked-by: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Jeff Mahoney <jeffm@...e.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Scott Norton <scott.norton@...com>
> Cc: Tom Vaden <tom.vaden@...com>
> Cc: Aswin Chandramouleeswaran <aswin@...com>
> Cc: Waiman Long <Waiman.Long@...com>
> Cc: Jason Low <jason.low2@...com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> ---
>  kernel/futex.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 577481d..af1fc31 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -69,6 +69,63 @@
>  
>  #include "locking/rtmutex_common.h"
>  
> +/*
> + * Basic futex operation and ordering guarantees:
> + *
> + * The waiter reads the futex value in user space and calls
> + * futex_wait(). It computes the hash bucket and acquires the hash

does                It
refer to "the waiter" or to futex_wait()?
I read it as referring to the waiter, but ISTM that the comments are using It
to refer to futex_wait()... ???

Thanks.

> + * bucket lock. After that it reads the futex user space value again
> + * and verifies that the data has not changed. If it has not changed
> + * it enqueues itself into the hash bucket, releases the hash
> + * bucket lock and schedules.
> + *
> + * The waker side modifies the user space value of the futex and calls
> + * futex_wake(). It computes the hash bucket and acquires the hash
> + * bucket lock. Then it looks for waiters on that futex in the hash
> + * bucket and wakes them.
> + *
> + * Note that the spin_lock serializes waiters and wakers, so that the
> + * following scenario is avoided:
> + *

-- 
~Randy
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