130 lines
4.3 KiB
Groff
130 lines
4.3 KiB
Groff
.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 18, 2001
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.Dt WITNESS 4
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.Os
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.Sh NAME
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.Nm witness
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.Nd lock validation facility
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.Sh SYNOPSIS
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.Cd options WITNESS
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.Cd options WITNESS_DDB
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.Cd options WITNESS_SKIPSPIN
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.Sh DESCRIPTION
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The
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.Nm
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module keeps track of the locks acquired and released by each thread.
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It also keeps track of the order in which locks are acquired with respect
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to each other.
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Each time a lock is acquired,
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.Nm
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uses these two lists to verify that a lock is not being acquired in the
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wrong order.
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If a lock order violation is detected, then a message is output to the
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kernel console detailing the locks involved and the locations in question.
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Witness can also be configured to drop into the kernel debugger when an order
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violation occurs.
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.Pp
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The
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.Nm
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code also checks various other conditions such as verifying that one
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does not recurse on a non-recursive lock.
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For sleep locks,
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.Nm
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verifies that a new process would not be switched to when a lock is released
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or a lock is blocked on during an acquire while any spin locks are held.
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If any of these checks fail, then the kernel will panic.
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.Pp
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The flag that controls whether or not the kernel debugger is entered when a
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lock order violation is detected can be set in a variety of ways.
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By default, the flag is off, but if the
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.Dv WITNESS_DDB
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kernel option is
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specified, then the flag will default to on.
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It can also be set from the
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.Xr loader 8
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via the
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.Va debug.witness_ddb
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environment variable or after the kernel has booted via the
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.Va debug.witness_ddb
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sysctl.
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If the flag is set to zero, then the debugger will not be entered.
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If the flag is non-zero, then the debugger will be entered.
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.Pp
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The
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.Nm
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code can also be configured to skip all checks on spin mutexes.
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By default, this flag defaults to off, but it can be turned on by
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specifying the
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.Dv WITNESS_SKIPSPIN
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kernel option.
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The flag can also be set via the
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.Xr loader 8
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environment variable
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.Va debug.witness.skipspin .
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If the variable is set to a non-zero value, then spin mutexes are skipped.
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Once the kernel has booted, the status of this flag can be examined but not
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set via the read-only sysctl
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.Va debug.witness_skipspin .
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.Pp
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The
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.Nm
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code also provides two extra
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.Xr ddb 4
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commands if both
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.Nm
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and
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.Xr ddb 4
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are compiled into the kernel:
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.Bl -ohang
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.It Ic show locks
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Outputs the list of locks held by the current thread to the kernel console
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along with the filename and line number at which each lock was last acquired
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by this thread.
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.It Ic show witness
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Dump the current order list to the kernel console.
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The code first displays the lock order tree for all of the sleep locks.
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Then it displays the lock order tree for all of the spin locks.
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Finally, it displays a list of locks that have not yet been acquired.
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.El
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.Sh SEE ALSO
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.Xr ddb 4 ,
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.Xr mutex 9
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.Sh HISTORY
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The
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.Nm
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code first appeared in
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.Tn BSD/OS
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5.0 and was imported from there into
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.Fx 5.0 .
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.Sh BUGS
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The
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.Nm
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code currently does not handle recursion of shared
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.Xr sx 9
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locks properly.
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