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From: Linus Torvalds <>
To: Kirill Smelkov <>
Cc: Rasmus Villemoes <>,
	Al Viro <>, Arnd Bergmann <>,
	Christoph Hellwig <>,
	Greg Kroah-Hartman <>,
	linux-fsdevel <>,
	Linux List Kernel Mailing <>
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	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, Apr 11, 2019 at 5:38 AM Kirill Smelkov <> 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

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

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


  reply	other threads:[~2019-04-11 16:23 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-26 22:20 [PATCH 1/3] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock 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]     ` <>
2019-04-24 19:16       ` Kirill Smelkov
     [not found] ` <>
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] ` <>
     [not found]   ` <>
     [not found]     ` <>
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

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