FreeBSD src
435068aab7
- In 8.x and above the run-queue locks are nomore shared even in the
HTT case, so remove the special case.
- The deadlock explained in the removed comment here is still possible
even with different locks, with the contribution of tdq_lock_pair().
An explanation is here:
(hypotesis: a thread needs to migrate on another CPU, thread1 is doing
sched_switch_migrate() and thread2 is the one handling the sched_switch()
request or in other words, thread1 is the thread that needs to migrate
and thread2 is a thread that is going to be preempted, most likely an
idle thread. Also, 'old' is referred to the context (in terms of
run-queue and CPU) thread1 is leaving and 'new' is referred to the
context thread1 is going into. Finally, thread3 is doing tdq_idletd()
or sched_balance() and definitively doing tdq_lock_pair())
* thread1 blocks its td_lock. Now td_lock is 'blocked'
* thread1 drops its old runqueue lock
* thread1 acquires the new runqueue lock
* thread1 adds itself to the new runqueue and sends an IPI_PREEMPT
through tdq_notify() to the new CPU
* thread1 drops the new lock
* thread3, scanning the runqueues, locks the old lock
* thread2 received the IPI_PREEMPT and does thread_lock() with td_lock
pointing to the new runqueue
* thread3 wants to acquire the new runqueue lock, but it can't because
it is held by thread2 so it spins
* thread1 wants to acquire old lock, but as long as it is held by
thread3 it can't
* thread2 going further, at some point wants to switchin in thread1,
but it will wait forever because thread1->td_lock is in blocked state
This deadlock has been manifested mostly on 7.x and reported several time
on mailing lists under the voice 'spinlock held too long'.
Many thanks to des@ for having worked hard on producing suitable textdumps
and Jeff for help on the comment wording.
Reviewed by: jeff
Reported by: des, others
Tested by: des, Giovanni Trematerra
<giovanni dot trematerra at gmail dot com>
(STABLE_7 based version)
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| bin | ||
| cddl | ||
| contrib | ||
| crypto | ||
| etc | ||
| games | ||
| gnu | ||
| include | ||
| kerberos5 | ||
| lib | ||
| libexec | ||
| release | ||
| rescue | ||
| sbin | ||
| secure | ||
| share | ||
| sys | ||
| tools | ||
| usr.bin | ||
| usr.sbin | ||
| COPYRIGHT | ||
| LOCKS | ||
| MAINTAINERS | ||
| Makefile | ||
| Makefile.inc1 | ||
| ObsoleteFiles.inc | ||
| README | ||
| UPDATING | ||
This is the top level of the FreeBSD source directory. This file was last revised on: $FreeBSD$ For copyright information, please see the file COPYRIGHT in this directory (additional copyright information also exists for some sources in this tree - please see the specific source directories for more information). The Makefile in this directory supports a number of targets for building components (or all) of the FreeBSD source tree, the most commonly used one being ``world'', which rebuilds and installs everything in the FreeBSD system from the source tree except the kernel, the kernel-modules and the contents of /etc. The ``world'' target should only be used in cases where the source tree has not changed from the currently running version. See: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/makeworld.html for more information, including setting make(1) variables. The ``buildkernel'' and ``installkernel'' targets build and install the kernel and the modules (see below). Please see the top of the Makefile in this directory for more information on the standard build targets and compile-time flags. Building a kernel is a somewhat more involved process, documentation for which can be found at: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig.html And in the config(8) man page. Note: If you want to build and install the kernel with the ``buildkernel'' and ``installkernel'' targets, you might need to build world before. More information is available in the handbook. The sample kernel configuration files reside in the sys/<arch>/conf sub-directory (assuming that you've installed the kernel sources), the file named GENERIC being the one used to build your initial installation kernel. The file NOTES contains entries and documentation for all possible devices, not just those commonly used. It is the successor of the ancient LINT file, but in contrast to LINT, it is not buildable as a kernel but a pure reference and documentation file. Source Roadmap: --------------- bin System/user commands. contrib Packages contributed by 3rd parties. crypto Cryptography stuff (see crypto/README). etc Template files for /etc. games Amusements. gnu Various commands and libraries under the GNU Public License. Please see gnu/COPYING* for more information. include System include files. kerberos5 Kerberos5 (Heimdal) package. lib System libraries. libexec System daemons. release Release building Makefile & associated tools. rescue Build system for statically linked /rescue utilities. sbin System commands. secure Cryptographic libraries and commands. share Shared resources. sys Kernel sources. tools Utilities for regression testing and miscellaneous tasks. usr.bin User commands. usr.sbin System administration commands. For information on synchronizing your source tree with one or more of the FreeBSD Project's development branches, please see: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/synching.html