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From:   Daeho Jeong <daeho43@gmail.com>
Date:   Fri, 12 Mar 2021 23:37:29 +0900
Message-ID: <CACOAw_yjyy+58B=RawAaQO98NQB43roZOv4sq5313sFHN1myXQ@mail.gmail.com>
Subject: Re: [PATCH v4] f2fs: add sysfs nodes to get runtime compression stat
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
        Daeho Jeong <daehojeong@google.com>
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As you can see, if we're doing like the below.

sbi->compr_written_block +=3D blocks;

Let's assume the initial value as 0.

<thread A>                                             <thread B>
sbi->compr_written_block =3D 0;

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

sbi->compr_written_block =3D 2;

Finally, we end up with 2, not 5.

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

2021=EB=85=84 3=EC=9B=94 12=EC=9D=BC (=EA=B8=88) =EC=98=A4=ED=9B=84 11:04, =
Greg KH <gregkh@linuxfoundation.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> 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 read=
er.
> >
> > +#define add_compr_block_stat(inode, blocks)                           =
 \
> > +       do {                                                           =
 \
> > +               struct f2fs_sb_info *sbi =3D F2FS_I_SB(inode);         =
   \
> > +               int diff =3D 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 c=
ondition.
>
> Why?  What are you trying to protect from "racing" here?
>
> thanks,
>
> greg k-h