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Re: [Xen-devel] [xen-unstable test] 13461: regressions - FAIL

Ian Campbell wrote:
On Thu, 2012-07-05 at 11:40 +0100, Ian Jackson wrote:
Daniel P. Berrange writes ("Re: [Xen-devel] [xen-unstable test] 13461: regressions - 
Yes, as you say flock() operates on the inode, so if something deletes
and recreates the file, future flocks will operate differently. Ideally
you should just never rm the files at all.

If you need to 'rm' them, then to avoid this, you must do two things

  - Only 'rm /foo' while holding the lock on /foo
  - Record the inode before acquiring the lock. After acquiring the
    lock check whether the inode on disk is the same. If not,
    release the lock&  repeat.
It seems more logical to me to check the inum of the open fd against
the file.  Something like this perhaps (untested):

diff -r ad08cd8e7097 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 11:39:59 2012 +0100
@@ -30,6 +30,7 @@ _setlockfd()
      let _lockfd=200+i
+    let _lockfile="$LOCK_BASEDIR/$1"

@@ -37,13 +38,32 @@ 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.
+    local rightfile
+    while true; do
+        eval "exec $_lockfd>>$lockfile"

you mean $_lockfile here I think.

+        flock -x $_lockfd
+        # We can't just stat /dev/stdin or /proc/self/fd/$_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 '

Won't this need to become $(PERL) (or @PERL@ and some seddery at install
time) for the benefit of BSD?

BSD don't use this scripts, so you don't have to worry about this here.

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