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[xen-unstable] hotplug/Linux: do not leak lockfiles



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patchbot at xen

Jul 5, 2012, 3:22 PM

Post #1 of 1 (104 views)
[xen-unstable] hotplug/Linux: do not leak lockfiles

# HG changeset patch
# User Ian Jackson <Ian.Jackson [at] eu>
# Date 1341488425 -3600
# Node ID 497e2fe4945594cd2846f0302bef5e5dafa1c2c6
# Parent ad08cd8e7097ec6da6526cf8ac26f0fa72cd1e01
hotplug/Linux: do not leak lockfiles

25590:bb250383a4f5 introduced a new locking scheme. Unfortunately it
leaves behind files in /var/run/xen-hotplug. These are spotted as
regressions by the autotester.

Fix this. This involves changing the locking protocol to allow
lockfiles to be deleted (as removing lockfiles is unsafe with a naive
flock-based algorithm).

Signed-off-by: Ian Jackson <ian.jackson [at] eu>
Committed-by: Ian Jackson <ian.jackson [at] eu>

diff -r ad08cd8e7097 -r 497e2fe49455 tools/hotplug/Linux/locking.sh
--- a/tools/hotplug/Linux/locking.sh Thu Jul 05 11:00:28 2012 +0100
+++ b/tools/hotplug/Linux/locking.sh Thu Jul 05 12:40:25 2012 +0100
@@ -30,6 +30,7 @@ _setlockfd()
let _lockfd=200+i
+ _lockfile="$LOCK_BASEDIR/$1"

@@ -37,13 +38,92 @@ claim_lock()
mkdir -p "$LOCK_BASEDIR"
_setlockfd $1
- eval "exec $_lockfd>>$LOCK_BASEDIR/$1"
- flock -x $_lockfd
+ # The locking strategy is identical to that from with-lock-ex(1)
+ # from chiark-utils, except using flock. It has the benefit of
+ # it being possible to safely remove the lockfile when done.
+ # See below for a correctness proof.
+ local rightfile
+ while true; do
+ eval "exec $_lockfd>>$_lockfile"
+ flock -x $_lockfd || return $?
+ # We can't just stat /dev/stdin or /proc/self/fd/$_lockfd or
+ # use bash's test -ef because those all go through what is
+ # actually a synthetic symlink in /proc and we aren't
+ # guaranteed that our stat(2) won't lose the race with an
+ # rm(1) between reading the synthetic link and traversing the
+ # file system to find the inum. Perl is very fast so use that.
+ rightfile=$( perl -e '
+ open STDIN, "<&'$_lockfd'" or die $!;
+ my $fd_inum = (stat STDIN)[1]; die $! unless defined $fd_inum;
+ my $file_inum = (stat $ARGV[0])[1];
+ print "y\n" if $fd_inum eq $file_inum;
+ ' "$_lockfile" )
+ if [ x$rightfile = xy ]; then break; fi
+ done

_setlockfd $1
- flock -u $_lockfd
+ rm "$_lockfile"
+# Protocol and correctness proof:
+# * The lock is owned not by a process but by an open-file (informally
+# an fd). Any process with an fd onto this open-file is a
+# lockholder and may perform the various operations; such a process
+# should only do so when its co-lockholder processes expect. Ie, we
+# will treat all processes holding fds onto the open-file as acting
+# in concert and not distinguish between them.
+# * You are a lockholder if
+# - You have an fd onto an open-file which
+# currently holds an exclusive flock lock on its inum
+# - and that inum is currently linked at the lockfile path
+# * The rules are:
+# - No-one but a lockholder may unlink the lockfile path
+# (or otherwise cause it to stop referring to a file it
+# refers to).
+# - Anyone may open the lockfile with O_CREAT
+# * The protocol for locking is:
+# - Open the file (O_CREAT)
+# - flock it
+# - fstat the fd you have open
+# - stat the lockfile path
+# - if both are equal you have the lock, otherwise try again.
+# * Informal proof of exclusivity:
+# - No two open-files can hold an fcntl lock onto the same file
+# at the same time
+# - No two files can have the same name at the same time
+# * Informal proof of correctness of locking protocol:
+# - After you call flock successfully no-one other than you
+# (someone with the same open-file) can stop you having
+# that flock lock.
+# - Obviously the inum you get from the fstat is fixed
+# - At the point where you call stat there are two
+# possibilities:
+# (i) the lockfile path referred to some other inum
+# in which case you have failed
+# (ii) the lockfile path referred to the same file
+# in which case at that point you were the
+# lockholder (by definition).
+# * Informal proof that no-one else can steal the lock:
+# - After you call flock successfully no-one other than you
+# can stop you having that flock lock
+# - No-one other than the lockholder is permitted to stop
+# the path referring to a particular inum. So if you
+# hold the lock then only you are allowed to stop the
+# path referring to the file whose flock you hold; so
+# it will continue to refer to that file.
+# That's both the conditions for being the lockholder.
+# Thus once you hold the lock at any instant, you will
+# continue to do so until you voluntarily stop doing so
+# (eg by unlinking the lockfile or closing the fd).

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