[Xen-devel] locking mechanism in hotplug scripts not working

[Marek Marczykowski <marmarek@xxxxxxxxxxxx>]

Hello,

I've found that locking isn't working as it should... It allows many
processes to claim lock simultaneously:

Full _claim_lock and description of race:
 48 _claim_lock()
 49 {
 50   local lockdir="$1"
 51   local owner=$(_lock_owner "$lockdir")
 52   local retries=0
 53
 54   while [ $retries -lt $LOCK_RETRIES ]
 55   do
 56     mkdir "$lockdir" 2>/dev/null && trap "_release_lock $lockdir;
sigerr" ERR &&
 57       _update_lock_info "$lockdir" && return
 58
 59     local new_owner=$(_lock_owner "$lockdir")
 60     if [ "$new_owner" != "$owner" ]
 61     then
 62       owner="$new_owner"
 63       retries=0
 64     else
 65       local pid=$(echo $owner | cut -d : -f 1)
 66       if [ -n "$pid" -a "$pid" != "unknown" -a ! -f
"/proc/$pid/status" ]
 67       then
 68         _release_lock $lockdir
 69       fi
 70     fi
 71
 72     if [ $retries -gt $LOCK_SPINNING_RETRIES ]
 73     then
 74       sleep $LOCK_SLEEPTIME
 75     else
 76       sleep 0
 77     fi
 78     retries=$(($retries + 1))
 79   done
 80   _steal_lock "$lockdir"
 81 }


The race is:
P1: claim_lock - success
P2: claim_lock - read lock owner ("owner" file in lock dir) - l51
P1: done things and release_lock
P1: exit
P3: claim_lock - success
P2: notice that P1 is dead (read previously) -> release_lock l68 (!!!)
P2: claim_lock l56 - success

Both P2 and P3 in critical section

I don't have idea how to fix it in current shape. Some workaround is to
remove lines 64-69... Perhaps proper fix is to use flock(1) utility, but
this will may be less portable...

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski         | RLU #390519
marmarek at mimuw edu pl   | xmpp:marmarek at staszic waw pl

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