linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-ID: <alpine.LFD.0.999.0708151941150.16414@enigma.security.iitk.ac.in>
Date:	Wed, 15 Aug 2007 20:05:38 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Chris Snook <csnook@...hat.com>, clameter@....com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	ak@...e.de, 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, segher@...nel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

Hi Stefan,


On Wed, 15 Aug 2007, Stefan Richter wrote:

> On 8/15/2007 10:18 AM, Heiko Carstens wrote:
> > On Wed, Aug 15, 2007 at 02:49:03PM +0800, Herbert Xu wrote:
> >> Chris Snook <csnook@...hat.com> wrote:
> >> > 
> >> > Because atomic operations are generally used for synchronization, which requires 
> >> > volatile behavior.  Most such codepaths currently use an inefficient barrier(). 
> >> >  Some forget to and we get bugs, because people assume that atomic_read() 
> >> > actually reads something, and atomic_write() actually writes something.  Worse, 
> >> > these are architecture-specific, even compiler version-specific bugs that are 
> >> > often difficult to track down.
> >> 
> >> I'm yet to see a single example from the current tree where
> >> this patch series is the correct solution.  So far the only
> >> example has been a buggy piece of code which has since been
> >> fixed with a cpu_relax.
> > 
> > Btw.: we still have
> > 
> > include/asm-i386/mach-es7000/mach_wakecpu.h:  while (!atomic_read(deassert));
> > include/asm-i386/mach-default/mach_wakecpu.h: while (!atomic_read(deassert));
> > 
> > Looks like they need to be fixed as well.
> 
> 
> I don't know if this here is affected:

Yes, I think it is. You're clearly expecting the read to actually happen
when you call get_hpsb_generation(). It's clearly not a busy-loop, so
cpu_relax() sounds pointless / wrong solution for this case, so I'm now
somewhat beginning to appreciate the motivation behind this series :-)

But as I said, there are ways to achieve the same goals of this series
without using "volatile".

I think I'll submit a RFC/patch or two on this myself (will also fix
the code pieces listed here).


> /* drivers/ieee1394/ieee1394_core.h */
> static inline unsigned int get_hpsb_generation(struct hpsb_host *host)
> {
> 	return atomic_read(&host->generation);
> }
> 
> /* drivers/ieee1394/nodemgr.c */
> static int nodemgr_host_thread(void *__hi)
> {
> 	[...]
> 
> 	for (;;) {
> 		[... sleep until bus reset event ...]
> 
> 		/* Pause for 1/4 second in 1/16 second intervals,
> 		 * to make sure things settle down. */
> 		g = get_hpsb_generation(host);
> 		for (i = 0; i < 4 ; i++) {
> 			if (msleep_interruptible(63) ||
> 			    kthread_should_stop())
> 				goto exit;

Totally unrelated, but this looks weird. IMHO you actually wanted:

	msleep_interruptible(63);
	if (kthread_should_stop())
		goto exit;

here, didn't you? Otherwise the thread will exit even when
kthread_should_stop() != TRUE (just because it received a signal),
and it is not good for a kthread to exit on its own if it uses
kthread_should_stop() or if some other piece of kernel code could
eventually call kthread_stop(tsk) on it.

Ok, probably the thread will never receive a signal in the first
place because it's spawned off kthreadd which ignores all signals
beforehand, but still ...

[PATCH] ieee1394: Fix kthread stopping in nodemgr_host_thread

The nodemgr host thread can exit on its own even when kthread_should_stop
is not true, on receiving a signal (might never happen in practice, as
it ignores signals). But considering kthread_stop() must not be mixed with
kthreads that can exit on their own, I think changing the code like this
is clearer. This change means the thread can cut its sleep short when
receive a signal but looking at the code around, that sounds okay (and
again, it might never actually recieve a signal in practice).

Signed-off-by: Satyam Sharma <satyam@...radead.org>

---

 drivers/ieee1394/nodemgr.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/ieee1394/nodemgr.c b/drivers/ieee1394/nodemgr.c
index 2ffd534..981a7da 100644
--- a/drivers/ieee1394/nodemgr.c
+++ b/drivers/ieee1394/nodemgr.c
@@ -1721,7 +1721,8 @@ static int nodemgr_host_thread(void *__hi)
 		 * to make sure things settle down. */
 		g = get_hpsb_generation(host);
 		for (i = 0; i < 4 ; i++) {
-			if (msleep_interruptible(63) || kthread_should_stop())
+			msleep_interruptible(63);
+			if (kthread_should_stop())
 				goto exit;
 
 			/* Now get the generation in which the node ID's we collect
-
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