linux-kernel - Re: [RFC PATCH] mutex: Documentation rewrite

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Date:	Wed, 21 May 2014 12:52:58 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	hpa@...or.com, waiman.long@...com, jason.low2@...com, aswin@...com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mutex: Documentation rewrite

On Wed, 2014-05-21 at 12:02 -0700, Tim Chen wrote:
> On Wed, 2014-05-21 at 10:41 -0700, Davidlohr Bueso wrote:
> > From: Davidlohr Bueso <davidlohr@...com>
> > 
> > Our mutexes have gone a long ways since the original implementation
> > back in 2005/2006. However, the mutex-design.txt document is still
> > stuck in the past, to the point where most of the information there
> > is practically useless and, more important, simply incorrect. This
> > patch pretty much rewrites it to resemble what we have nowadays.
> > 
> > Since regular semaphores are almost much extinct in the kernel
> > (most users now rely on mutexes or rwsems), it no longer makes
> > sense to have such a close comparison, which was copied from most
> > of the cover letter when Ingo introduced the generic mutex subsystem.
> > 
> > While users who really want to learn more about kernel mutexes can
> > go and read the code, I have marked this RFC since I'm sure more gory
> > details can be included. Also, ww_mutexes are intentionally left out,
> > leaving things as generic as possible.
> > 
> > Comments welcome!
> 
> Thanks for doing this.
> 
> > +
> > +(iii) slowpath: last resource, if the lock is still unable to be acquired
> 
> change "last resource" to "last resort"

Ops, yes, that's what I meant.

> >  
> >  Disadvantages
> >  -------------
> >  
> > -The stricter mutex API means you cannot use mutexes the same way you
> > -can use semaphores: e.g. they cannot be used from an interrupt context,
> > -nor can they be unlocked from a different context that which acquired
> > -it. [ I'm not aware of any other (e.g. performance) disadvantages from
> > -using mutexes at the moment, please let me know if you find any. ]
> > -
> 
> Should we keep this instead of deleting it?  It is still true we
> cannot use mutex from an interrupt context.

Well I wouldn't necessarily classify 'cannot be used from interrupt
context' as something bad, it's just a characteristic of mutexes. Since
we mention it in the Semantics part I thought we can remove it.

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