linux-ext4 - Re: [PATCH][RFC] ext4: avoid taking down_read(&grp->alloc

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Date:	Mon, 14 Feb 2011 20:18:11 +0200
From:	"Amir G." <amir73il@...rs.sourceforge.net>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Theodore Tso <tytso@....edu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH][RFC] ext4: avoid taking down_read(&grp->alloc_sem)

On Mon, Feb 14, 2011 at 6:30 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> On 2011-02-14, at 0:52, "Amir G." <amir73il@...rs.sourceforge.net> wrote:
>>> @@ -1160,7 +1160,15 @@ ext4_mb_load_buddy(struct super_block *sb,
>>>
>>> +       /*
>>> +        * We only need to take the read lock if other groups share the buddy
>>> +        * page with this group or if blocks may be added to this (last) group
>>> +        * by ext4_group_extend().
>>> +        */
>>> +       if (blocks_per_page > 2 || group == sbi->s_groups_count - 1)
>>> +               e4b->alloc_semp = &grp->alloc_sem;
>
> No comment on whether this change is safe or not, but shouldn't this check be:
>
>        if (blocks_per_page > 1 ||
>

No, it should be groups_per_page > 1, as Aneesh suggested,
which translates to blocks_per_group > 2, but this is obviously not
clear from the code...
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