From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alberich de megres <alberich2k5@gmail.com>
Subject: RB tree
Date: Wed, 10 Mar 2010 12:07:24 +0100
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Hi!

I know this could be a stupid question, but taking a look at rbtree
kernel imprelantacion used in ext3 fs, I understand what lines do, but
i could not see what the use it (i'm not telling it is pointless, only
that i didn't understand why its doing what its doing)

I can see it's zeroing last 2 bits on the rb_parent_color:
#define rb_parent(r)   ((struct rb_node *)((r)->rb_parent_color & ~3))

We are getting only last bit:
#define rb_color(r)   ((r)->rb_parent_color & 1)

Setting or clearing the last bit:
#define rb_set_red(r)  do { (r)->rb_parent_color &= ~1; } while (0)
#define rb_set_black(r)  do { (r)->rb_parent_color |= 1; } while (0)


And here, comes my trouble:
static inline void rb_set_parent(struct rb_node *rb, struct rb_node *p)
{
         rb->rb_parent_color = (rb->rb_parent_color & 3) | (unsigned long)p;
}
static inline void rb_set_color(struct rb_node *rb, int color)
{
        rb->rb_parent_color = (rb->rb_parent_color & ~1) | color;
}
we are aligning the pointer address??


Thanks for the patience!!

Alberich