linux-kernel - Re: [PATCH v4] f2fs: add sysfs nodes to get runtime compression stat

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Date:   Fri, 12 Mar 2021 23:37:29 +0900
From:   Daeho Jeong <daeho43@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [PATCH v4] f2fs: add sysfs nodes to get runtime compression stat

As you can see, if we're doing like the below.

sbi->compr_written_block += blocks;

Let's assume the initial value as 0.

<thread A>                                             <thread B>
sbi->compr_written_block = 0;

sbi->compr_written_block = 0;
+blocks(3);
                                                               + blocks(2);
sbi->compr_written_block = 3;

sbi->compr_written_block = 2;

Finally, we end up with 2, not 5.

As more threads are participating it, we might miss more counting.

2021년 3월 12일 (금) 오후 11:04, Greg KH <gregkh@...uxfoundation.org>님이 작성:
>
> On Fri, Mar 12, 2021 at 10:56:13PM +0900, Daeho Jeong wrote:
> > Thanks for suggesting me sysfs_emit().
> >
> > For atomic values, actually, those are needed for writer part, not reader.
> >
> > +#define add_compr_block_stat(inode, blocks)                            \
> > +       do {                                                            \
> > +               struct f2fs_sb_info *sbi = F2FS_I_SB(inode);            \
> > +               int diff = F2FS_I(inode)->i_cluster_size - blocks;      \
> > +               atomic64_add(blocks, &sbi->compr_written_block);        \
> > +               atomic64_add(diff, &sbi->compr_saved_block);            \
> > +       } while (0)
> >
> > I needed a protection here, because they might be updated in the race condition.
>
> Why?  What are you trying to protect from "racing" here?
>
> thanks,
>
> greg k-h