linux-kernel - RE: [PATCH AUTOSEL 5.0 59/66] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock

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Date:   Thu, 25 Apr 2019 10:04:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kirill Smelkov' <kirr@...edi.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
CC:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        "Yongzhi Pan" <panyongzhi@...il.com>,
        Jonathan Corbet <corbet@....net>,
        David Vrabel <david.vrabel@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Miklos Szeredi <miklos@...redi.hu>, Tejun Heo <tj@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Arnd Bergmann <arnd@...db.de>, Christoph Hellwig <hch@....de>,
        Julia Lawall <Julia.Lawall@...6.fr>,
        Nikolaus Rath <Nikolaus@...h.org>,
        Han-Wen Nienhuys <hanwen@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH AUTOSEL 5.0 59/66] fs: stream_open - opener for
 stream-like files so that read and write can run simultaneously without
 deadlock

From: Kirill Smelkov
> Sent: 24 April 2019 19:30
> 
> On Wed, Apr 24, 2019 at 10:26:55AM -0700, Linus Torvalds wrote:
> > On Wed, Apr 24, 2019 at 10:19 AM Sasha Levin <sashal@...nel.org> wrote:
> > >
> > > Hm, I might be confusing something here but I see a bunch of patches
> > > that convert existing callers mentioned in this patch to use
> > > stream_open() which was introduced here.
> >
> > The only use of stream_open() upstream right now is the xenbus
> > conversion, and that isn't actually a bugfix, because xenbus used to
> > manually do that
> >
> >         filp->f_mode &= ~FMODE_ATOMIC_POS; /* cdev-style semantics */
> >
> > that stream_open() does.
> >
> > So no, there isn't "a bunch of patches" anywhere.
> >
> > There are *future* cleanups for 5.2 that will happen, and that might
> > have hit linux-next. And there is at least one FUSE patch (again -
> > pending, not upstream) that may get marked for stable.
> >
> > But I see nothing right now that makes it stable material yet.
> 
> Linus, thanks for explaining. Sasha, Greg, there is a FUSE patch that
> should be stable material that will need this stream_open() thing. That
> patch has just entered fuse.git#for-next today:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?id=bbd84f33652f
> 
> and will hopefully enter 5.2 when merge window opens. I agree we should
> not blindly backport bulk stream_open conversions as performed by
> stream_open.cocci, at least unless there is a bug report indicating that
> it is actually required for a particular driver. On the other hand both
> Xen and FUSE deadlocks were hit for real which justifies stable
> propagation for their fixes.
> 
> You can read about the deadlock regression and the plan to fix it in
> original "fs: stream_open - opener for stream-like files so that read
> and write can run simultaneously without deadlock" patch (the 59/66
> patch that was send in this thread), or here:
> 
> 	https://git.kernel.org/linus/10dce8af3422
> 
> 
> Hope it clarifies things a bit,

I can also imagine drivers that expect accesses to be done using
pread() and pwrite() - maybe only if the fd is shared.
Provided accesses get the correct offset they can be concurrent.
In fact they only need to update the offset in the file structure
when they complete - they may do this already.

I know (I think) uclibc implementing pread() as lseek() + read()
caused me grief - but that might just have been the extra system
call overhead rather than any problems with the offset.

	David

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