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Linus Torvalds 2be7d348fe Revert "vfs: properly and reliably lock f_pos in fdget_pos()" 
This reverts commit 0be0ee7181.

I was hoping it would be benign to switch over entirely to FMODE_STREAM,
and we'd have just a couple of small fixups we'd need, but it looks like
we're not quite there yet.

While it worked fine on both my desktop and laptop, they are fairly
similar in other respects, and run mostly the same loads.  Kenneth
Crudup reports that it seems to break both his vmware installation and
the KDE upower service.  In both cases apparently leading to timeouts
due to waitinmg for the f_pos lock.

There are a number of character devices in particular that definitely
want stream-like behavior, but that currently don't get marked as
streams, and as a result get the exclusion between concurrent
read()/write() on the same file descriptor.  Which doesn't work well for
them.

The most obvious example if this is /dev/console and /dev/tty, which use
console_fops and tty_fops respectively (and ptmx_fops for the pty master
side).  It may be that it's just this that causes problems, but we
clearly weren't ready yet.

Because there's a number of other likely common cases that don't have
llseek implementations and would seem to act as stream devices:

  /dev/fuse		(fuse_dev_operations)
  /dev/mcelog		(mce_chrdev_ops)
  /dev/mei0		(mei_fops)
  /dev/net/tun		(tun_fops)
  /dev/nvme0		(nvme_dev_fops)
  /dev/tpm0		(tpm_fops)
  /proc/self/ns/mnt	(ns_file_operations)
  /dev/snd/pcm*		(snd_pcm_f_ops[])

and while some of these could be trivially automatically detected by the
vfs layer when the character device is opened by just noticing that they
have no read or write operations either, it often isn't that obvious.

Some character devices most definitely do use the file position, even if
they don't allow seeking: the firmware update code, for example, uses
simple_read_from_buffer() that does use f_pos, but doesn't allow seeking
back and forth.

We'll revisit this when there's a better way to detect the problem and
fix it (possibly with a coccinelle script to do more of the FMODE_STREAM
annotations).

Reported-by: Kenneth R. Crudup <kenny@panix.com>
Cc: Kirill Smelkov <kirr@nexedi.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2019-11-26 11:34:06 -08:00
arch Merge branch 'x86-iopl-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 11:12:02 -08:00
block for-5.5/disk-revalidate-20191122 2019-11-25 11:37:01 -08:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto Merge git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2019-11-25 19:49:58 -08:00
Documentation Merge branch 'x86-asm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 10:42:40 -08:00
drivers Merge branch 'x86-hyperv-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 09:43:34 -08:00
fs Revert "vfs: properly and reliably lock f_pos in fdget_pos()" 2019-11-26 11:34:06 -08:00
include Revert "vfs: properly and reliably lock f_pos in fdget_pos()" 2019-11-26 11:34:06 -08:00
init Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
ipc ipc/sem.c: convert to use built-in RCU list checking 2019-09-25 17:51:41 -07:00
kernel Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
lib Merge branch 'x86-asm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 10:42:40 -08:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
samples Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next 2019-11-20 18:11:23 -08:00
scripts Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
security Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2019-11-25 20:02:57 -08:00
sound spi: Updates for v5.5 2019-11-25 21:32:37 -08:00
tools Merge branch 'x86-iopl-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 11:12:02 -08:00
usr kbuild: update compile-test header list for v5.4-rc2 2019-10-05 15:29:49 +09:00
virt KVM: Fix jump label out_free_* in kvm_init() 2019-11-23 11:29:17 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Modules updates for v5.4 2019-09-22 10:34:46 -07:00
.mailmap ARM: SoC fixes 2019-11-10 13:41:59 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer 2019-10-10 08:12:51 -07:00
Kbuild kbuild: do not descend to ./Kbuild when cleaning 2019-08-21 21:03:58 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS Merge branches 'core-objtool-for-linus', 'x86-cleanups-for-linus' and 'x86-apic-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-26 08:21:54 -08:00
Makefile Linux 5.4 2019-11-24 16:32:01 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.