[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Mon, 3 Mar 2014 13:56:31 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
lkml <linux-kernel@...r.kernel.org>,
Miklos Szeredi <miklos@...redi.hu>,
"Theodore T'so" <tytso@....edu>, Christoph Hellwig <hch@....de>,
Chris Mason <clm@...com>, Dave Chinner <david@...morbit.com>,
Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
"J. Bruce Fields" <bfields@...i.umich.edu>,
Yongzhi Pan <panyongzhi@...il.com>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O
On Mon, Mar 3, 2014 at 1:45 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Um... That's odd - we *could* get there with f.need_put and
> file_count(file) equal to 1, but why would we want to take
> f_pos_lock in that case?
Because that means that the file table is shared among threads. So
another thread can access the struct file pointer and do a concurrent
read() or write() on it, and so we need to lock f_pos.
Basically, there are two cases:
- duplicated file pointers due to fork(). That's the "file_count()" test.
Yes, this will trigger even if they didn't fork, just dup'ed the
file descriptor. We have no way of telling the difference, though.
- concurrent access due to duplicated "struct file_table" pointers.
This is the "need_put" test, since __fget_light() will have tested the
proper files->count already.
Both need f_pos_lock.
Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists