From mboxrd@z Thu Jan  1 00:00:00 1970
From: tobias@gambas-buch.de (Tobias Boege)
Date: Tue, 29 Jan 2013 16:56:02 +0100
Subject: thread concurrent file operation
In-Reply-To: <CAEEuMqf5cH-bVYSJd0MWTvxSYPwijX6f9GuYc6HnsauD5MK5PQ@mail.gmail.com>
References: <CAEEuMqf5cH-bVYSJd0MWTvxSYPwijX6f9GuYc6HnsauD5MK5PQ@mail.gmail.com>
Message-ID: <20130129155602.GA991@aurora>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Tue, 29 Jan 2013, Karaoui mohamed lamine wrote:
> Hello,
> 
> I was looking at how a syscall read/write was done, and i found this :
> 
>    ....
>    loff_t pos = file_pos_read(f.file);
>    ret = vfs_read(f.file, buf, count, &pos);
>    file_pos_write(f.file, pos);
>    fdput(f);
>    ...
> 
> My questions are :
> 
> Where did the locking go? I would have imaginated something like :
> 
>    ....
>    *lock(f);*
>    loff_t pos = file_pos_read(f.file);
>    ret = vfs_read(f.file, buf, count, &pos);
>    file_pos_write(f.file, pos);
>    fdput(f);
>    *unlock(f);*
>    ...
> 
> If multiple threads try to read/write at the same time, they could
> read/write at the same offset ?
> 

Look some lines above:

	struct fd f = fdget(fd);

Regards,
Tobi