linux-kernel - Re: [PATCH 1/3] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock

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Date:   Thu, 11 Apr 2019 09:22:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kirill Smelkov <kirr@...edi.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] fs: stream_open - opener for stream-like files so
 that read and write can run simultaneously without deadlock

On Thu, Apr 11, 2019 at 5:38 AM Kirill Smelkov <kirr@...edi.com> wrote:
>
> However file->f_pos writing is still there and it will bug under race
> detector, e.g. under KTSAN because read and write can be running
> simultaneously. Maybe it is not only race bug, but also a bit of
> slowdown as read and write code paths write to the same memory thus
> needing inter-cpu synchronization if read and write handlers are on
> different cpus. However for this I'm not sure.

I doubt it's noticeable, but it would probably be good to simply not
do the write for streams.

That *could* be done somewhat similarly, with just changing th address
to be updated, or course.

In fact, we *used* to (long ago) pass in the address of "file->f_pos"
itself to the low-level read/write routines. We then changed it to do
that indirection through a local copy of pos (and
file_pos_read/file_pos_write) because we didn't do the proper locking,
so different read/write versions could mess with each other (and with
lseek).

But one of the things that commit 9c225f2655e36 ("vfs: atomic f_pos
accesses as per POSIX") did was to add the proper locking at least for
the cases that we care about deeply, so we *could* say that we have
three cases:

 - FMODE_ATOMIC_POS: properly locked,
 - FMODE_STREAM: no pos at all
 - otherwise a "mostly don't care - don't mix!"

and so we could go back to not copying the pos at all, and instead do
something like

        loff_t *ppos = f.file->f_mode & FMODE_STREAM ? NULL : &file->f_pos;
        ret = vfs_write(f.file, buf, count, ppos);

and perhaps have a long-term plan to try to get rid of the "don't mix"
case entirely (ie "if you use f_pos, then we'll do the proper
locking")

(The above is obviously surrounded by the fdget_pos()/fdput_pos() that
implements the locking decision).

           Linus