linux-kernel - Re: [PATCH 9/24] make atomic_read() behave consistently on ia64

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Message-ID: <46BCE9F8.4060009@redhat.com>
Date:	Fri, 10 Aug 2007 18:43:04 -0400
From:	Chris Snook <csnook@...hat.com>
To:	"Luck, Tony" <tony.luck@...el.com>
CC:	Andreas Schwab <schwab@...e.de>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, akpm@...ux-foundation.org, ak@...e.de,
	heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com
Subject: Re: [PATCH 9/24] make atomic_read() behave consistently on ia64

Luck, Tony wrote:
>> Use atomic64_read to read an atomic64_t.
> 
> Thanks Andreas!
> 
> Chris: This bug is why the 8-byte loads got changed to 4-byte + sign-extend
> by your change to atomic_read().

I figured as much.  Thanks for confirming this.

> With this applied together with shuffling the volatile from the
> declaration to the usage (in both atomic_read() and atomic_set()
> the generated code *almost* reverts to the original.
> 
> There are some differences where ld4 have turned into ld8 though.
> Are these bugs in the use of atomic_add() and atomic_sub().  E.g.
> the first of these changes is in: ipc/msg.c:freeque() where we have:
> 
> 	atomic_sub(msg->q_cbytes, &msg_bytes);
> 
> Now the type of msg->q_cbytes is "unsigned long" ... so it seems a
> poor idea to subtract such a large typed object from "msg_bytes"
> which is a mere slip of an atomic_t.
> 
> Or is there some other type-wrangling that needs to happen in
> include/asm-ia64/atomic.h?  There are a total of nineteen of
> these ld4->ld8 transforms.

Possibly.  Either that or we've uncovered some latent bugs.  Maybe a 
combination of the two.  Can you list those 19 changes so we can 
evaluate them?  I'm told there were some *(volatile *) bugs fixed in gcc 
recently, so it's also possible your 3.4.6 is showing those.  I can test 
that on a more recent gcc on ia64 if it's inconvenient for you to do so 
on your test box.

	-- Chris
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