Linux-Fsdevel Archive on lore.kernel.org
 help / color / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Kirill Smelkov <kirr@nexedi.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Al Viro <viro@zeniv.linux.org.uk>, Arnd Bergmann <arnd@arndb.de>,
	Christoph Hellwig <hch@lst.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux List Kernel Mailing <linux-kernel@vger.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
Date: Thu, 11 Apr 2019 09:22:56 -0700
Message-ID: <CAHk-=whJtZt52SnhBGrNMnuxFn3GE9X_e02x8BPxtkqrfyZukw@mail.gmail.com> (raw)
In-Reply-To: <20190411123816.GA18309@deco.navytux.spb.ru>

On Thu, Apr 11, 2019 at 5:38 AM Kirill Smelkov <kirr@nexedi.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

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-26 22:20 Kirill Smelkov
2019-03-26 23:22 ` [PATCH 3/3] fuse: Add FOPEN_STREAM and use stream_open() if filesystem returned that from open handler Kirill Smelkov
2019-04-24  7:13   ` [RESEND, PATCH " Kirill Smelkov
     [not found]     ` <20190424160611.2A71321900@mail.kernel.org>
2019-04-24 19:16       ` Kirill Smelkov
     [not found] ` <8794193f3040b798010970228d978c05ad56ec52.1553637462.git.kirr@nexedi.com>
2019-03-27  6:54   ` [PATCH 2/3] *: convert stream-like files from nonseekable_open -> stream_open Lubomir Rintel
2019-03-27 16:58 ` [PATCH 1/3] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Juergen Gross
2019-04-06 17:07 ` Linus Torvalds
2019-04-07 20:04   ` Kirill Smelkov
2019-04-08  0:09     ` Linus Torvalds
2019-04-14  7:11       ` Kirill Smelkov
     [not found] ` <4c4651e2-167e-bfcc-7b3e-cda118f98a69@rasmusvillemoes.dk>
     [not found]   ` <20190409203807.GA13855@deco.navytux.spb.ru>
     [not found]     ` <d8c23d05-8810-13a2-cc50-7a47ff35e90b@rasmusvillemoes.dk>
2019-04-11 12:38       ` Kirill Smelkov
2019-04-11 16:22         ` Linus Torvalds [this message]
2019-04-12 12:42           ` Kirill Smelkov
2019-04-13 16:54             ` Kirill Smelkov
2019-04-13 16:54               ` [PATCH 1/2] vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files Kirill Smelkov
2019-04-13 17:27                 ` Linus Torvalds
2019-04-13 17:38                   ` Al Viro
2019-04-13 18:44                   ` Kirill Smelkov
2019-04-13 16:55               ` [PATCH 2/2] vfs: use &file->f_pos directly on files that have position Kirill Smelkov

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAHk-=whJtZt52SnhBGrNMnuxFn3GE9X_e02x8BPxtkqrfyZukw@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=arnd@arndb.de \
    --cc=gregkh@linuxfoundation.org \
    --cc=hch@lst.de \
    --cc=kirr@nexedi.com \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux@rasmusvillemoes.dk \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fsdevel/0 linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ https://lore.kernel.org/linux-fsdevel \
		linux-fsdevel@vger.kernel.org
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fsdevel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git