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From: David Laight <David.Laight@ACULAB.COM>
To: 'Kirill Smelkov' <kirr@nexedi.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Sasha Levin <sashal@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Linux List Kernel Mailing" <linux-kernel@vger.kernel.org>,
	stable <stable@vger.kernel.org>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	"Yongzhi Pan" <panyongzhi@gmail.com>,
	Jonathan Corbet <corbet@lwn.net>,
	David Vrabel <david.vrabel@citrix.com>,
	Juergen Gross <jgross@suse.com>,
	Miklos Szeredi <miklos@szeredi.hu>, Tejun Heo <tj@kernel.org>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Arnd Bergmann <arnd@arndb.de>, Christoph Hellwig <hch@lst.de>,
	Julia Lawall <Julia.Lawall@lip6.fr>,
	Nikolaus Rath <Nikolaus@rath.org>,
	Han-Wen Nienhuys <hanwen@google.com>,
	linux-fsdevel <linux-fsdevel@vger.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
Date: Fri, 26 Apr 2019 11:00:01 +0000	[thread overview]
Message-ID: <073e5def9e654a1d80cdd79cdcf23361@AcuMS.aculab.com> (raw)
In-Reply-To: <20190426074522.GA16247@deco.navytux.spb.ru>

From: Kirill Smelkov
> Sent: 26 April 2019 08:46
...
> I'm not sure I understand your comment completely, but we convert to
> stream_open only drivers that actually do _not_ use position at all, and
> that were already using nonseekable_open, thus pread and pwrite were
> already returning -ESPIPE for them (nonseekable_open clears
> FMODE_{PREAD,PWRITE} and ksys_{pread,pwrite}64 check for that flag). We
> also convert only drivers that use no_llseek for .llseek, so lseek
> on those files is/was always returning -ESPIPE as well.
> 
> If a driver uses position in its read and write and has support for
> pread/pwrite (FMODE_PREAD and FMODE_PWRITE), pread and pwrite are
> already working _without_ file->f_pos locking - because those system
> calls do not semantically update file->f_pos at all and thus do not take
> file->f_pos_lock - i.e. pread/pwrite can be run simultaneously already.

Looks like I knew that once :-)
Mind you, 'man pread' on my system is somewhat uninformative.

Maybe pread() should always be allowed at offset 0.
Then you wouldn't need all this extra logic.

> If libc implements pread as lseek+read it will work for a single
> user case  (single thread, or fd not shared between processes), but it
> will break because of lseek+read non-atomicity if multiple preads are
> simultaneously used from several threads. And also for such emulation
> for multiple users case there is a chance for pread vs pwrite deadlock,
> since those system calls are using read and write and read and write
> take file->f_pos_lock.

I'd actually rather the pread() failed to compile.
The actual implementation did 3 lseek()s (to save and restore the offset).
A user level emulation could usually get away with one lseek().

	David

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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  reply	other threads:[~2019-04-26 11:00 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190424143341.27665-1-sashal@kernel.org>
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 56/66] kernel/sysctl.c: fix out-of-bounds access when setting file-max Sasha Levin
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 59/66] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Sasha Levin
2019-04-24 16:34   ` Greg Kroah-Hartman
2019-04-24 16:40     ` Linus Torvalds
2019-04-24 17:02       ` Greg Kroah-Hartman
2019-04-24 17:19       ` Sasha Levin
2019-04-24 17:26         ` Linus Torvalds
2019-04-24 18:30           ` Kirill Smelkov
2019-04-25 10:04             ` David Laight
2019-04-26  7:45               ` Kirill Smelkov
2019-04-26 11:00                 ` David Laight [this message]
2019-04-26 18:20                   ` Kirill Smelkov
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 62/66] pin iocb through aio Sasha Levin
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 63/66] aio: fold lookup_kiocb() into its sole caller Sasha Levin
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 64/66] aio: keep io_event in aio_kiocb Sasha Levin
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 65/66] aio: store event at final iocb_put() Sasha Levin
2019-04-24 14:33 ` [PATCH AUTOSEL 5.0 66/66] Fix aio_poll() races Sasha Levin

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