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Manuel Bouyer
6e2d9b317d
On NetBSD, some block device configuration requires serialisation. Introcuce locking functions (derived from the Linux version), and use them in the block script where appropriate. Signed-off-by: Manuel Bouyer <bouyer@netbsd.org> Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
122 lines
4.3 KiB
Bash
122 lines
4.3 KiB
Bash
#
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# Copyright (c) 2005 XenSource Ltd.
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# Copyright (c) 2007 Red Hat
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#
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# This library is free software; you can redistribute it and/or
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# modify it under the terms of version 2.1 of the GNU Lesser General Public
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# License as published by the Free Software Foundation.
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#
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# This library is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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# Lesser General Public License for more details.
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#
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# You should have received a copy of the GNU Lesser General Public
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# License along with this library; If not, see <http://www.gnu.org/licenses/>.
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#
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#
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# Serialisation
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#
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LOCK_BASEDIR=$XEN_LOCK_DIR/xen-hotplug
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_setlockfd()
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{
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_lockfd=9
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_lockfile="$LOCK_BASEDIR/$1"
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}
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claim_lock()
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{
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mkdir -p "$LOCK_BASEDIR"
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_setlockfd $1
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# The locking strategy is identical to that from with-lock-ex(1)
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# from chiark-utils, except using flock. It has the benefit of
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# it being possible to safely remove the lockfile when done.
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# See below for a correctness proof.
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local stat
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while true; do
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eval "exec $_lockfd<> $_lockfile"
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# we can't flock $_lockfd here, as the shell closes it on exec.
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# Workaround by redirecting to 0 for the command, and flock 0 instead.
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flock -v -x 0 0<& $_lockfd|| exit 1
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local file_stat
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local fd_stat
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if fd_stat=$(stat -f '%d.%i' 0<&$_lockfd 2>/dev/null) && file_stat=$(stat -f '%d.%i' $_lockfile 2>/dev/null )
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then
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if [ "$fd_stat" = "$file_stat" ] ; then break; fi
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fi
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# Some versions of bash appear to be buggy if the same
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# $_lockfile is opened repeatedly. Close the current fd here.
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eval "exec $_lockfd<&-"
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done
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}
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release_lock()
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{
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_setlockfd $1
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rm "$_lockfile"
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}
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# Protocol and correctness proof:
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#
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# * The lock is owned not by a process but by an open-file (informally
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# an fd). Any process with an fd onto this open-file is a
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# lockholder and may perform the various operations; such a process
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# should only do so when its co-lockholder processes expect. Ie, we
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# will treat all processes holding fds onto the open-file as acting
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# in concert and not distinguish between them.
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#
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# * You are a lockholder if
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# - You have an fd onto an open-file which
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# currently holds an exclusive flock lock on its inum
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# - and that inum is currently linked at the lockfile path
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#
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# * The rules are:
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# - No-one but a lockholder may unlink the lockfile path
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# (or otherwise cause it to stop referring to a file it
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# refers to).
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# - Anyone may open the lockfile with O_CREAT
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#
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# * The protocol for locking is:
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# - Open the file (O_CREAT)
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# - flock it
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# - fstat the fd you have open
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# - stat the lockfile path
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# - if both are equal you have the lock, otherwise try again.
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#
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# * Informal proof of exclusivity:
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# - No two open-files can hold an fcntl lock onto the same file
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# at the same time
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# - No two files can have the same name at the same time
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#
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# * Informal proof of correctness of locking protocol:
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# - After you call flock successfully no-one other than you
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# (someone with the same open-file) can stop you having
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# that flock lock.
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# - Obviously the inum you get from the fstat is fixed
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# - At the point where you call stat there are two
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# possibilities:
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# (i) the lockfile path referred to some other inum
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# in which case you have failed
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# (ii) the lockfile path referred to the same file
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# in which case at that point you were the
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# lockholder (by definition).
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#
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# * Informal proof that no-one else can steal the lock:
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# - After you call flock successfully no-one other than you
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# can stop you having that flock lock
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# - No-one other than the lockholder is permitted to stop
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# the path referring to a particular inum. So if you
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# hold the lock then only you are allowed to stop the
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# path referring to the file whose flock you hold; so
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# it will continue to refer to that file.
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# That's both the conditions for being the lockholder.
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#
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# Thus once you hold the lock at any instant, you will
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# continue to do so until you voluntarily stop doing so
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# (eg by unlinking the lockfile or closing the fd).
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