ext2/3: Incomplete/buggy use of s_debts in Orlov

[Stephen C. Tweedie]

Hi all,

I've been tidying up the online resize code for -mm and come across one
nasty item.  The s_debts array is the only dynamic, fs-sized array in
the entirety of ext2/3, so it's the only dynamic array we need to grow
when we resize.

But to get the locking for that right, we need to know what the locking
for s_debts is in the first place.  Looking at that I found two rather
odd things:

On ext2, s_debts is modified with only the per-group lock:

	spin_lock(sb_bgl_lock(sbi, group));
	if (S_ISDIR(mode)) {
		if (sbi->s_debts[group] < 255)
			sbi->s_debts[group]++;

where s_debts is defined as

	u8 *s_debts;

in the superblock.  That was OK when we had a global lock_super(sb)
round every allocation, but it's extremely dangerous to perform byte
accesses like this when the rest of the memory word is shared with other
CPUs.  It certainly risks word-tearing on machines like Alpha; I think
it's theoretically unsafe even on x86.

On ext3, the situation is actually much simpler:

s_debts is never modified at all.

Ever.

It's allocated, carefully checked, then freed.  But never modified.  

Now, that's fine with me --- it makes the locking for s_debts *really*
easy to get right. :-)   And in my current resize patches, I just nuke
s_debts entirely.  

But if anybody thinks it really needs to stay, then a Plan B is
required, and we probably need to start off by fixing the locking in
ext2 itself.

--Stephen

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