freebsd with flexible iflib nic queues
47ad691f87
Specifically, a critical_exit() call that drops the nesting level to zero has a brief window where the pending preemption flag is set and the nesting level is set to zero. This is done purposefully to avoid races where a preemption scheduled by an interrupt could be lost otherwise (see revision 144777). However, this does mean that if an interrupt fires during this window and enters and exits a critical section, it may preempt from the interrupt context. This is generally fine as the interrupt code is careful to arrange critical sections so that they are not exited until it is safe to preempt (e.g. interrupts EOI'd and masked if necessary). However, the SMP rendezvous IPI handler does not quite follow this rule, and in general a rendezvous can never be preempted. Rendezvous handlers are also not permitted to schedule threads to execute, so they will not typically trigger preemptions. SMP rendezvous handlers may use spinlocks (carefully) such as the rm_cleanIPI() handler used in rmlocks, but using a spinlock also enters and exits a critical section. If the interrupted top-half code is in the brief window of critical_exit() where the nesting level is zero but a preemption is pending, then releasing the spinlock can trigger a preemption. Because we know that SMP rendezvous handlers can never schedule a thread, we know that a critical_exit() in an SMP rendezvous handler will only preempt in this edge case. We also know that the top-half thread will happily handle the deferred preemption once the SMP rendezvous has completed, so the preemption will not be lost. This makes it safe to employ a workaround where we use a nested critical section in the SMP rendezvous code itself around rendezvous action routines to prevent any preemptions during an SMP rendezvous. The workaround intentionally avoids checking for a deferred preemption when leaving the critical section on the assumption that if there is a pending preemption it will be handled by the interrupted top-half code. Submitted by: mlaier (variation specific to rm_cleanIPI()) Obtained from: Isilon MFC after: 1 week |
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| 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 | ||
| Makefile.mips | ||
| 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. cddl Various commands and libraries under the Common Development and Distribution License. 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