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

[Randy Dunlap <rdunlap@infradead.org>]

On 12/19/13 12:45, Davidlohr Bueso wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> That's essential, if you want to hack on futexes.
> 
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Darren Hart <dvhart@linux.intel.com>
> Acked-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Jeff Mahoney <jeffm@suse.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Scott Norton <scott.norton@hp.com>
> Cc: Tom Vaden <tom.vaden@hp.com>
> Cc: Aswin Chandramouleeswaran <aswin@hp.com>
> Cc: Waiman Long <Waiman.Long@hp.com>
> Cc: Jason Low <jason.low2@hp.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Davidlohr Bueso <davidlohr@hp.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