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Linus Torvalds d8e464ecc1 vfs: mark pipes and sockets as stream-like file descriptors 
In commit 3975b097e5 ("convert stream-like files -> stream_open, even
if they use noop_llseek") Kirill used a coccinelle script to change
"nonseekable_open()" to "stream_open()", which changed the trivial cases
of stream-like file descriptors to the new model with FMODE_STREAM.

However, the two big cases - sockets and pipes - don't actually have
that trivial pattern at all, and were thus never converted to
FMODE_STREAM even though it makes lots of sense to do so.

That's particularly true when looking forward to the next change:
getting rid of FMODE_ATOMIC_POS entirely, and just using FMODE_STREAM to
decide whether f_pos updates are needed or not.  And if they are, we'll
always do them atomically.

This came up because KCSAN (correctly) noted that the non-locked f_pos
updates are data races: they are clearly benign for the case where we
don't care, but it would be good to just not have that issue exist at
all.

Note that the reason we used FMODE_ATOMIC_POS originally is that only
doing it for the minimal required case is "safer" in that it's possible
that the f_pos locking can cause unnecessary serialization across the
whole write() call.  And in the worst case, that kind of serialization
can cause deadlock issues: think writers that need readers to empty the
state using the same file descriptor.

[ Note that the locking is per-file descriptor - because it protects
  "f_pos", which is obviously per-file descriptor - so it only affects
  cases where you literally use the same file descriptor to both read
  and write.

  So a regular pipe that has separate reading and writing file
  descriptors doesn't really have this situation even though it's the
  obvious case of "reader empties what a bit writer concurrently fills"

  But we want to make pipes as being stream-line anyway, because we
  don't want the unnecessary overhead of locking, and because a named
  pipe can be (ab-)used by reading and writing to the same file
  descriptor. ]

There are likely a lot of other cases that might want FMODE_STREAM, and
looking for ".llseek = no_llseek" users and other cases that don't have
an lseek file operation at all and making them use "stream_open()" might
be a good idea.  But pipes and sockets are likely to be the two main
cases.

Cc: Kirill Smelkov <kirr@nexedi.com>
Cc: Eic Dumazet <edumazet@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Marco Elver <elver@google.com>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Paul McKenney <paulmck@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2019-11-25 09:12:11 -08:00
arch arm64 fix for 5.4 2019-11-21 12:15:24 -08:00
block iocost: check active_list of all the ancestors in iocg_activate() 2019-11-14 13:56:54 -07:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto Merge branch 'next-lockdown' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2019-09-28 08:14:15 -07:00
Documentation Merge branch 'x86-pti-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-11-12 10:53:24 -08:00
drivers virtio: last minute bugfixes 2019-11-23 13:02:18 -08:00
fs vfs: mark pipes and sockets as stream-like file descriptors 2019-11-25 09:12:11 -08:00
include Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2019-11-22 14:28:14 -08:00
init Merge branch 'next-lockdown' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2019-09-28 08:14:15 -07:00
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 2019-11-22 14:28:14 -08:00
lib lib/xz: fix XZ_DYNALLOC to avoid useless memory reallocations 2019-11-15 18:34:00 -08:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm mm/ksm.c: don't WARN if page is still mapped in remove_stable_node() 2019-11-22 09:11:18 -08:00
net vfs: mark pipes and sockets as stream-like file descriptors 2019-11-25 09:12:11 -08:00
samples samples/bpf: fix build by setting HAVE_ATTR_TEST to zero 2019-10-31 21:39:15 +01:00
scripts arm64 fix for -rc8 / final 2019-11-15 09:14:23 -08:00
security efi/efi_test: Lock down /dev/efi_test and require CAP_SYS_ADMIN 2019-10-31 09:40:21 +01:00
sound ALSA: usb-audio: Fix incorrect size check for processing/extension units 2019-11-14 18:01:22 +01:00
tools GPIO fixes for v5.4: 2019-11-21 12:01:30 -08:00
usr kbuild: update compile-test header list for v5.4-rc2 2019-10-05 15:29:49 +09:00
virt KVM: Add a comment describing the /dev/kvm no_compat handling 2019-11-15 10:14:04 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig
.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 git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2019-11-22 14:28:14 -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.