linux-kernel - Re: smp_read_barrier

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Message-ID: <20160103180830.GA32217@linux.vnet.ibm.com>
Date:	Sun, 3 Jan 2016 10:08:30 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Petko Manolov <petkan@...-labs.com>
Cc:	mingo@...nel.org, realmz6@...il.com,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: smp_read_barrier_depends() for Blackfin

On Sun, Jan 03, 2016 at 06:25:39PM +0200, Petko Manolov wrote:
>  Content preview:  Hi Paul, Ingo, It seems to me that smp_read_barrier_depends
>     (which resolves to ___raw_smp_check_barrier_asm) is overdoing it, unless
>    that particular part it is written for is a disaster in terms of cache coherency.
>     [...] 
> 
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> 	Hi Paul, Ingo,
> 
> It seems to me that smp_read_barrier_depends (which resolves to 
> ___raw_smp_check_barrier_asm) is overdoing it, unless that particular part it is 
> written for is a disaster in terms of cache coherency.
> 
> So far this is the only architecture that i know of (baring Alpha) which employs 
> non-empty read_barrier_depends().  I am wondering if this is really needed or 
> those who did the arch port got overly enthusiastic.  If it is the former then 
> you may include another example of crazy architecture in your book. :)

Hello, Petko,

I must defer to the architecture maintainers.  That said, there was a time
when the blackfin maintainer were trying to make an SMP system without
cache coherence, that is, by simply wiring two UP-only blackfin CPUs
into a single system.  They were using cache-flush tricks to make things
more-or-less work.  And their ___raw_smp_check_barrier_asm() does look
to be flushing caches, so maybe that is what is happening here.  And the
comment header for read_barrier_depends() seems to support this view.

Adding the blackfin folks and the usual lists on CC.  Might get
us something better than my half-remembered hearsay and possible
misinterpretations of header comments.  ;-)

That said, cache-incoherent systems might well be a good addition to
the book.

							Thanx, Paul

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