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Linus Torvalds 6551d5c56e pipe: make sure to wake up everybody when the last reader/writer closes Andrei Vagin reported that commit `0ddad21d3e` ("pipe: use exclusive waits when reading or writing") broke one of the CRIU tests. He even has a trivial reproducer: #include <unistd.h> #include <sys/types.h> #include <sys/wait.h> int main() { int p[2]; pid_t p1, p2; int status; if (pipe(p) == -1) return 1; p1 = fork(); if (p1 == 0) { close(p[1]); read(p[0], &status, sizeof(status)); return 0; } p2 = fork(); if (p2 == 0) { close(p[1]); read(p[0], &status, sizeof(status)); return 0; } sleep(1); close(p[1]); wait(&status); wait(&status); return 0; } and the problem - once he points it out - is obvious. We use these nice exclusive waits, but when the last writer goes away, it then needs to wake up _every_ reader (and conversely, the last reader disappearing needs to wake every writer, of course). In fact, when going through this, we had several small oddities around how to wake things. We did in fact wake every reader when we changed the size of the pipe buffers. But that's entirely pointless, since that just acts as a possible source of new space - no new data to read. And when we change the size of the buffer, we don't need to wake all writers even when we add space - that case acts just as if somebody made space by reading, and any writer that finds itself not filling it up entirely will wake the next one. On the other hand, on the exit path, we tried to limit the wakeups with the proper poll keys etc, which is entirely pointless, because at that point we obviously need to wake up everybody. So don't do that: just wake up everybody - but only do that if the counts changed to zero. So fix those non-IO wakeups to be more proper: space change doesn't add any new data, but it might make room for writers, so it wakes up a writer. And the actual changes to reader/writer counts should wake up everybody, since everybody is affected (ie readers will all see EOF if the writers have gone away, and writers will all get EPIPE if all readers have gone away). Fixes: `0ddad21d3e` ("pipe: use exclusive waits when reading or writing") Reported-and-tested-by: Andrei Vagin <avagin@gmail.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Matthew Wilcox <willy@infradead.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>		2020-02-18 14:34:36 -08:00
arch	Bugfixes and improvements to selftests. On top of this, Mauro converted the	2020-02-16 13:01:42 -08:00
block	block-5.6-2020-02-05	2020-02-06 06:15:23 +00:00
certs	certs: Add wrapper function to check blacklisted binary hash	2019-11-12 12:25:50 +11:00
crypto	s390 updates for 5.6-rc2	2020-02-15 13:10:38 -08:00
Documentation	Minor bug fixes for IPMI	2020-02-16 13:05:46 -08:00
drivers	Minor bug fixes for IPMI	2020-02-16 13:05:46 -08:00
fs	pipe: make sure to wake up everybody when the last reader/writer closes	2020-02-18 14:34:36 -08:00
include	Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip	2020-02-15 12:51:22 -08:00
init	Various fixes:	2020-02-11 16:39:18 -08:00
ipc	proc: convert everything to "struct proc_ops"	2020-02-04 03:05:26 +00:00
kernel	Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip	2020-02-15 12:51:22 -08:00
lib	Various fixes:	2020-02-11 16:39:18 -08:00
LICENSES	LICENSES: Rename other to deprecated	2019-05-03 06:34:32 -06:00
mm	Merge branch 'merge.nfs-fs_parse.1' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs	2020-02-08 13:26:41 -08:00
net	NFS Client Bugfixes for Linux 5.6-rc2	2020-02-14 14:46:11 -08:00
samples	Kbuild updates for v5.6 (2nd)	2020-02-09 16:05:50 -08:00
scripts	kbuild: fix mismatch between .version and include/generated/compile.h	2020-02-11 05:18:40 +09:00
security	selinux/stable-5.6 PR 20200210	2020-02-10 16:51:35 -08:00
sound	ALSA: pcm: Fix double hw_free calls	2020-02-13 16:30:22 +01:00
tools	Bugfixes and improvements to selftests. On top of this, Mauro converted the	2020-02-16 13:01:42 -08:00
usr	Kbuild updates for v5.6 (2nd)	2020-02-09 16:05:50 -08:00
virt	KVM: Disable preemption in kvm_get_running_vcpu()	2020-02-12 12:19:35 +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: use 'dts' diff driver for dts files	2019-12-04 19:44:11 -08:00
.gitignore	modpost: dump missing namespaces into a single modules.nsdeps file	2019-11-11 20:10:01 +09:00
.mailmap	A handful of small documentation fixes that wandered in.	2020-02-07 13:03:10 -08:00
COPYING
CREDITS	open: introduce openat2(2) syscall	2020-01-18 09:19:18 -05:00
Kbuild	kbuild: rename hostprogs-y/always to hostprogs/always-y	2020-02-04 01:53:07 +09:00
Kconfig	docs: kbuild: convert docs to ReST and rename to *.rst	2019-06-14 14:21:21 -06:00
MAINTAINERS	eCryptfs fixes for 5.6-rc3	2020-02-17 21:08:37 -08:00
Makefile	Linux 5.6-rc2	2020-02-16 13:16:59 -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.