linux-kernel - Re: [PATCH v2 1/3] rtmutex: update rt-mutex-design

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Message-ID: <20170421154752.n54qikxldxzi4hs2@hirez.programming.kicks-ass.net>
Date:   Fri, 21 Apr 2017 17:47:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alex Shi <alex.shi@...aro.org>
Cc:     mathieu.poirier@...aro.org, Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:LOCKING PRIMITIVES" <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/3] rtmutex: update rt-mutex-design

On Fri, Apr 21, 2017 at 10:12:53PM +0800, Alex Shi wrote:
> diff --git a/Documentation/locking/rt-mutex-design.txt b/Documentation/locking/rt-mutex-design.txt
> index 8666070..11beb55 100644
> --- a/Documentation/locking/rt-mutex-design.txt
> +++ b/Documentation/locking/rt-mutex-design.txt
> @@ -97,9 +97,9 @@ waiter   - A waiter is a struct that is stored on the stack of a blocked
>             a process being blocked on the mutex, it is fine to allocate
>             the waiter on the process's stack (local variable).  This
>             structure holds a pointer to the task, as well as the mutex that
> -           the task is blocked on.  It also has the plist node structures to
> -           place the task in the waiter_list of a mutex as well as the
> -           pi_list of a mutex owner task (described below).
> +	   the task is blocked on.  It also has a rbtree node structures to
> +	   place the task in waiters rbtree of a mutex as well as the
> +	   pi_waiters rbtree of a mutex owner task (described below).

whitespace fail

>  
>             waiter is sometimes used in reference to the task that is waiting
>             on a mutex. This is the same as waiter->task.
> @@ -179,53 +179,35 @@ again.
>                           |
>                     F->L5-+
>  
> +If the G process has highest priority in the chain, then all the tasks up
> +the chain (A and B in this example), must have their priorities increased
> +to that of G.

No, only the top task that's actually runnable needs to be modified. The
rest we don't care about because they're blocked.

> +Since the pi_waiters of a task holds an order by priority of all the top waiters
> +of all the mutexes that the task owns, rt_mutex_getprio simply needs to compare
> +the top pi waiter to its own normal priority, and return the higher priority
> +back.

rt_mutex_getprio() doesn't exist.

> +The main operation of this function is summarized by Thomas Gleixner in
> +rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
> +details.

Since all the useful bits are there anyway, why keep this document
around at all?