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From: Jan Kara <jack@suse.cz>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Eric Sandeen <esandeen@redhat.com>,
	Dave Chinner <dchinner@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: the "read" syscall sees partial effects of the "write" syscall
Date: Fri, 18 Sep 2020 15:13:17 +0200	[thread overview]
Message-ID: <20200918131317.GH18920@quack2.suse.cz> (raw)
In-Reply-To: <alpine.LRH.2.02.2009180509370.19302@file01.intranet.prod.int.rdu2.redhat.com>

On Fri 18-09-20 08:25:28, Mikulas Patocka wrote:
> I'd like to ask about this problem: when we write to a file, the kernel 
> takes the write inode lock. When we read from a file, no lock is taken - 
> thus the read syscall can read data that are halfway modified by the write 
> syscall.
> 
> The standard specifies the effects of the write syscall are atomic - see 
> this:
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_07

Yes, but no Linux filesystem (except for XFS AFAIK) follows the POSIX spec
in this regard. Mostly because the mixed read-write performance sucks when
you follow it (not that it would absolutely have to suck - you can use
clever locking with range locks but nobody does it currently). In practice,
the read-write atomicity works on Linux only on per-page basis for buffered
IO.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  reply	other threads:[~2020-09-18 13:13 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-15 12:34 [RFC] nvfs: a filesystem for persistent memory Mikulas Patocka
2020-09-15 13:00 ` Matthew Wilcox
2020-09-15 13:24   ` Mikulas Patocka
2020-09-22 10:04   ` Ritesh Harjani
2020-09-15 15:16 ` Dan Williams
2020-09-15 16:58   ` Mikulas Patocka
2020-09-15 17:38     ` Mikulas Patocka
2020-09-16 10:57       ` [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache Mikulas Patocka
2020-09-16 16:21         ` Dan Williams
2020-09-16 17:24           ` Mikulas Patocka
2020-09-16 17:40             ` Dan Williams
2020-09-16 18:06               ` Mikulas Patocka
2020-09-21 16:20                 ` NVFS XFS metadata (was: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache) Mikulas Patocka
2020-09-22  5:03                   ` Dave Chinner
2020-09-22 16:46                     ` Mikulas Patocka
2020-09-22 17:25                       ` Matthew Wilcox
2020-09-24 15:00                         ` Mikulas Patocka
2020-09-28 15:22                           ` Mikulas Patocka
2020-09-23  2:45                       ` Dave Chinner
2020-09-23  9:20                         ` A bug in ext4 with big directories (was: NVFS XFS metadata) Mikulas Patocka
2020-09-23  9:44                           ` Jan Kara
2020-09-23 12:46                             ` Mikulas Patocka
2020-09-23 20:20                             ` Andreas Dilger
2020-09-23 17:19                         ` NVFS XFS metadata (was: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache) Mikulas Patocka
2020-09-23  9:57                       ` Jan Kara
2020-09-23 13:11                         ` Mikulas Patocka
2020-09-23 15:04                           ` Matthew Wilcox
2020-09-22 12:28                   ` Matthew Wilcox
2020-09-22 12:39                     ` Mikulas Patocka
2020-09-16 18:56               ` [PATCH] pmem: fix __copy_user_flushcache Mikulas Patocka
2020-09-18  1:53                 ` Dan Williams
2020-09-18 12:25                   ` the "read" syscall sees partial effects of the "write" syscall Mikulas Patocka
2020-09-18 13:13                     ` Jan Kara [this message]
2020-09-18 18:02                       ` Linus Torvalds
2020-09-20 23:41                       ` Dave Chinner
2020-09-17  6:50               ` [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache Christoph Hellwig
2020-09-21 16:19   ` [RFC] nvfs: a filesystem for persistent memory Mikulas Patocka
2020-09-21 16:29     ` Dan Williams
2020-09-22 15:43     ` Ira Weiny

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