Hi Stefan,


On Wed, 15 Aug 2007, Stefan Richter wrote:

On 8/15/2007 10:18 AM, Heiko Carstens wrote:

quoted

On Wed, Aug 15, 2007 at 02:49:03PM +0800, Herbert Xu wrote:

quoted

Chris Snook [off-list ref] wrote:

quoted

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 <redacted>

---

 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