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Message-ID: <661de9470603131932h7ff99aacgde3911ae82b77dc8@mail.gmail.com>
Date: Tue, 14 Mar 2006 09:02:25 +0530
From: "Balbir Singh" <bsingharora@gmail.com>
To: "Nick Piggin" <nickpiggin@yahoo.com.au>
Subject: Re: [patch 1/3] radix tree: RCU lockless read-side
Cc: "Nick Piggin" <npiggin@suse.de>,
       "Linux Kernel" <linux-kernel@vger.kernel.org>,
       "Linux Memory Management" <linux-mm@kvack.org>
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> The "slots" member is an array, not an RCU assigned pointer. As such, after
> doing rcu_dereference(slot), you can access slot->slots[i] without further
> memory barriers I think?
>
> But I agree that code now is a bit inconsistent. I've cleaned things up a
> bit in my tree now... but perhaps it is easier if you send a patch to show
> what you mean (because sometimes I'm a bit dense, I'm afraid).
>

Fro starters, I do not think your dense at all.

Hmm... slot/slots is quite confusing name. I was referring to slot and
ended up calling it slots. The point I am contending is that
rcu_derefence(slot->slots[i]) should happen.

<snippet>
+                       __s = rcu_dereference(slot->slots[i]);
+                       if (__s != NULL)
                              break;
</snippet>

If we break from the loop because __s != NULL. Then in the snippet below

<snippet>
       /* Bottom level: grab some items */
      for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) {
              index++;
              if (slot->slots[i]) {
-                       results[nr_found++] = slot->slots[i];
+                       results[nr_found++] = &slot->slots[i];
                      if (nr_found == max_items)
                              goto out;
              }
</snippet>

We do not use __s above. "slot->slots[i]" is not rcu_derefenced() in
this case because we broke out of the loop above with __s being not
NULL. Another issue is - is it good enough to rcu_derefence() slot
once? Shouldn't all uses of *slot->* be rcu derefenced?

<suggestion (WARNING: patch has spaces and its not compiled)>
       /* Bottom level: grab some items */
      for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) {
              index++;
-              if (slot->slots[i]) {
-                       results[nr_found++] = slot->slots[i];
+             __s = rcu_dereference(slot->slots[i]);
+             if (__s) {
+                    /* This is tricky, cannot take the address of __s
or rcu_derefence() */
+                    results[nr_found++] = &slot->slots[i];
                      if (nr_found == max_items)
                              goto out;
              }
</suggestion>

I hope I am making sense.

Warm Regards,
Balbir
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