1b9254f885
This is useful in environments where system configuration is performed by automated interaction with the system console, since unexpected witness output makes such automation difficult. With this change, the new debug.witness.output_channel sysctl allows one to specify that witness output is to be printed to the kernel log (using log(9)) rather than the console. Reviewed by: cem, jhb MFC after: 2 weeks Relnotes: yes Sponsored by: EMC / Isilon Storage Division Differential Revision: https://reviews.freebsd.org/D4183
188 lines
5.6 KiB
Groff
188 lines
5.6 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 November 18, 2015
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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_COUNT
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.Cd options WITNESS_KDB
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.Cd options WITNESS_NO_VNODE
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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 or log detailing the locks involved and the locations in
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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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or attempt an upgrade on a shared lock held by another thread.
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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
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.Dv WITNESS_COUNT
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kernel option controls the maximum number of
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.Xr witness 4
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entries that are tracked in the kernel.
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The maximum number of entries can be queried via the
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.Va debug.witness.count
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sysctl.
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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.count
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environment variable.
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.Pp
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The
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.Dv WITNESS_NO_VNODE
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kernel option tells
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.Xr witness 4
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to ignore locking issues between
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.Xr vnode 9
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objects.
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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_KDB
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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.kdb
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environment variable or after the kernel has booted via the
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.Va debug.witness.kdb
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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 sysctl
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.Va debug.witness.watch
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specifies the level of witness involvement in the system.
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A value of 1 specifies that witness is enabled.
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A value of 0 specifies that witness is disabled, but that can be enabled
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again. This will maintain a small amount of overhead in the system.
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A value of -1 specifies that witness is disabled permanently and
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cannot be enabled again.
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The sysctl
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.Va debug.witness.watch
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can be set via
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.Xr loader 8 .
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.Pp
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The sysctl
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.Va debug.witness.output_channel
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specifies the output channel used to display warnings emitted by
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.Nm .
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The possible values are
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.Ql console ,
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indicating that warnings are to be printed to the system console,
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.Ql log ,
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indicating that warnings are to be logged via
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.Xr log 9 ,
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and
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.Ql none .
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This sysctl can be set via
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.Xr loader 8 .
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.Pp
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The
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.Nm
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code also provides three 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 Op thread
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Outputs the list of locks held by a 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 the thread.
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The optional
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.Ar thread
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argument may be either a TID,
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PID,
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or pointer to a thread structure.
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If
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.Ar thread
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is not specified,
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then the locks held by the current thread are displayed.
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.It Ic show all locks
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Outputs the list of locks held by all threads in the system to the
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kernel console.
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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 loader 8 ,
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.Xr sysctl 8 ,
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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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.Bsx 5.0
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and was imported from there into
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.Fx 5.0 .
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