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Waiman Long a9e9bcb45b locking/rwsem: Prevent decrement of reader count before increment
During my rwsem testing, it was found that after a down_read(), the
reader count may occasionally become 0 or even negative. Consequently,
a writer may steal the lock at that time and execute with the reader
in parallel thus breaking the mutual exclusion guarantee of the write
lock. In other words, both readers and writer can become rwsem owners
simultaneously.

The current reader wakeup code does it in one pass to clear waiter->task
and put them into wake_q before fully incrementing the reader count.
Once waiter->task is cleared, the corresponding reader may see it,
finish the critical section and do unlock to decrement the count before
the count is incremented. This is not a problem if there is only one
reader to wake up as the count has been pre-incremented by 1.  It is
a problem if there are more than one readers to be woken up and writer
can steal the lock.

The wakeup was actually done in 2 passes before the following v4.9 commit:

  70800c3c0c ("locking/rwsem: Scan the wait_list for readers only once")

To fix this problem, the wakeup is now done in two passes
again. In the first pass, we collect the readers and count them.
The reader count is then fully incremented. In the second pass, the
waiter->task is then cleared and they are put into wake_q to be woken
up later.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Fixes: 70800c3c0c ("locking/rwsem: Scan the wait_list for readers only once")
Link: http://lkml.kernel.org/r/20190428212557.13482-2-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-05-07 08:46:46 +02:00
arch Merge branch 'ras-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 19:54:57 -07:00
block
certs
crypto
Documentation * amd64_edac: Family 0x17, models 0x30-.. enablement (Yazen Ghannam) 2019-05-06 19:53:11 -07:00
drivers Merge branch 'ras-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 19:54:57 -07:00
fs Merge branch 'core-stacktrace-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 13:11:48 -07:00
include * amd64_edac: Family 0x17, models 0x30-.. enablement (Yazen Ghannam) 2019-05-06 19:53:11 -07:00
init x86/mm: Initialize PGD cache during mm initialization 2019-05-05 20:32:46 +02:00
ipc
kernel locking/rwsem: Prevent decrement of reader count before increment 2019-05-07 08:46:46 +02:00
lib Merge branch 'core-stacktrace-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 13:11:48 -07:00
LICENSES
mm arm64 updates for 5.2 2019-05-06 17:54:22 -07:00
net Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 14:50:46 -07:00
samples
scripts Merge branch 'core-objtool-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 11:39:17 -07:00
security s390 updates for the 5.2 merge window 2019-05-06 16:42:54 -07:00
sound ACPI updates for 5.2-rc1 2019-05-06 19:35:13 -07:00
tools ACPI updates for 5.2-rc1 2019-05-06 19:35:13 -07:00
usr
virt arm64 updates for 5.2 2019-05-06 17:54:22 -07:00
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore
.mailmap
COPYING
CREDITS
Kbuild
Kconfig
MAINTAINERS Merge branch 'ras-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 19:54:57 -07:00
Makefile Linux 5.1 2019-05-05 17:42:58 -07:00
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.