linux-kernel - Re: [PATCH 1/2] ext4: use little-endian bitops directly

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Date:	Tue, 31 May 2011 12:45:15 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Ted Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/2] ext4: use little-endian bitops directly

2011/5/31 Andreas Dilger <adilger@...ger.ca>:

> Looking more closely at these operations, ext4_set_bit_atomic() is not used anywhere in the code and could be removed.
>
> There is only one place where ext4_clear_bit_atomic() is used (ext4_add_groupblocks()), and this is likely incorrect.  None of the other ext4 code is using ext4_set_bit_atomic(), and all of the ext2_clear_bit_atomic() macros are silently ignoring the "lock" argument, so these changes are done without ext4_group_lock() being held for that group.  I suspect this is never a problem in normal usage because ext4_add_groupblocks() is only used during filesystem resizing, which is rare, and doubly rare for any allocations to be made in the same group concurrently.
>
> This one usage of ext4_clear_bit_atomic() should just be moved inside the ext4_lock_group() a few lines down and then use ext4_clear_bit() for consistency, and ext4_clear_bit_atomic() can be removed entirely.
>
> A further cleanup would be to change this whole function to use mb_clear_bits(), which is not only faster because it operates on many bits at once, but also doesn't require that the buddy bitmap be marked invalid ("NEED_INIT") after these changes are made, but that is work for a separate patch.

Looks like this is already done by the commit
e73a347b7723757bb5fb5c502814dc205a7f496d ("ext4: implement
ext4_add_groupblocks() by freeing blocks")

So we can remove ext4_{set,clear}_bit_atomic now.

> I would also encourage you to finish off this patch series by pushing up the generic ext2_{set,clear}_bit_atomic() to the few places that are currently using ext2_{set,clear}_bit_atomic() directly (looks like only fs/nilfs2/alloc.h and include/linux/ext3.h) and then removing them from the arch headers.

The difficulty of doing this is that there are two different
implementations of ext2_{set,clear}_bit_atomic (spin lock version and
test_and_{set,clear}_bit_le version).  If we can switch to one of which
on all architectures, the change is easy.  But I don't have less messy idea
to keep current behavior on all architectures.
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