Re: [Xen-devel] [PATCH] tools/hotplug: fix locking

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-06-20 at 18:53 +0100, Zhigang Wang wrote:
> On 06/20/2012 11:34 AM, Ian Campbell wrote:
> > On Wed, 2012-06-13 at 18:29 +0100, Zhigang Wang wrote:
> >> # HG changeset patch
> >> # User Zhigang Wang <zhigang.x.wang@xxxxxxxxxx>
> >> # Date 1339608534 14400
> >> # Node ID 650b03f412143c3057abbd0ae0e256e6b3fd2ba8
> >> # Parent  32034d1914a607d7b6f1f060352b4cac973600f8
> >> tools/hotplug: fix locking
> > A better title would be "tools/hotplug: Switch to locking with flock"
> > since "fix" is not very descriptive.
> Agree.
> >
> > The commit message should also state why changing this scheme is
> > preferable to fixing the current one.
> I have two points:
> 
> 1. No timeout: in the old one, if one process holding the lock more than 100
>    seconds, other processes could steal it.
> 2. No leftovers: if a process holding this lock is dead, it will close the 
> lock
>    file, so it will not affect other processes.
> 
> >
> > Is this flock tool widely available? Does it need to be added to the
> > dependencies in the README?
> It is widely distributed: it's part of the util-linux package. So I think no
> need to document it.
> >
> >> The current locking implementation would allow two processes get the lock
> >> simultaneously:
> > [...snip shell trace...]
> >
> > Can you add a line or two of analysis to explain where in that shell
> > spew things are actually going wrong and why?
> I didn't spent much time on this complicated implement. But it fails for me 
> and
> also for others (from response).
> >
> >> This patch is ported from Red Hat Enterprise Linux 5.8.
> > I think we need a Signed-off-by from the original patch author. (Unless
> > DCO clause b somehow applies?)
> I'm not sure how to handle this. There's no signed-off-by on the original Red
> Hat patch. Could anyone in Red Hat help to get it signed off?

Perhaps Andrew Jones or Don Dutile can point us in the right direction
(both CCd) if you identify the precise source of the patch (i.e. the
name of the .src.rpm and the name of the patch within it)?

I looked in kernel-2.6.18-308.8.2.el5.src.rpm (which seems fairly
recent) but maybe I'm looking in the wrong place.

Ian.
> 
> Thanks,
> 
> Zhigang
> >
> >
> > Thanks,
> > Ian.
> >
> >> Signed-off-by: Zhigang Wang <zhigang.x.wang@xxxxxxxxxx>
> >> Cc: Kouya Shimura <kouya@xxxxxxxxxxxxxx>
> >>
> >> diff -r 32034d1914a6 -r 650b03f41214 tools/hotplug/Linux/locking.sh
> >> --- a/tools/hotplug/Linux/locking.sh       Thu Jun 07 19:46:57 2012 +0100
> >> +++ b/tools/hotplug/Linux/locking.sh       Wed Jun 13 13:28:54 2012 -0400
> >> @@ -1,5 +1,6 @@
> >>  #
> >>  # Copyright (c) 2005 XenSource Ltd.
> >> +# Copyright (c) 2007 Red Hat
> >>  #
> >>  # This library is free software; you can redistribute it and/or
> >>  # modify it under the terms of version 2.1 of the GNU Lesser General 
> >> Public
> >> @@ -19,92 +20,30 @@
> >>  # Serialisation
> >>  #
> >>  
> >> -LOCK_SLEEPTIME=1
> >> -LOCK_SPINNING_RETRIES=5
> >> -LOCK_RETRIES=100
> >>  LOCK_BASEDIR=/var/run/xen-hotplug
> >>  
> >> +_setlockfd()
> >> +{
> >> +    local i
> >> +    for ((i = 0; i < ${#_lockdict}; i++))
> >> +    do [ -z "${_lockdict[$i]}" -o "${_lockdict[$i]}" = "$1" ] && break
> >> +    done
> >> +    _lockdict[$i]="$1"
> >> +    let _lockfd=200+i
> >> +}
> >> +
> >>  
> >>  claim_lock()
> >>  {
> >> -  local lockdir="$LOCK_BASEDIR/$1"
> >> -  mkdir -p "$LOCK_BASEDIR"
> >> -  _claim_lock "$lockdir"
> >> +    mkdir -p "$LOCK_BASEDIR"
> >> +    _setlockfd $1
> >> +    eval "exec $_lockfd>>$LOCK_BASEDIR/$1"
> >> +    flock -x $_lockfd
> >>  }
> >>  
> >>
> >>  release_lock()
> >>  {
> >> -  _release_lock "$LOCK_BASEDIR/$1"
> >> +    _setlockfd $1
> >> +    flock -u $_lockfd
> >>  }
> >> -
> >> -
> >> -# This function will be redefined in xen-hotplug-common.sh.
> >> -sigerr() {
> >> -  exit 1
> >> -}
> >> -
> >> -
> >> -_claim_lock()
> >> -{
> >> -  local lockdir="$1"
> >> -  local owner=$(_lock_owner "$lockdir")
> >> -  local retries=0
> >> -
> >> -  while [ $retries -lt $LOCK_RETRIES ]
> >> -  do
> >> -    mkdir "$lockdir" 2>/dev/null && trap "_release_lock $lockdir; sigerr" 
> >> ERR &&
> >> -      _update_lock_info "$lockdir" && return
> >> -
> >> -    local new_owner=$(_lock_owner "$lockdir")
> >> -    if [ "$new_owner" != "$owner" ]
> >> -    then
> >> -      owner="$new_owner"
> >> -      retries=0
> >> -    else
> >> -      local pid=$(echo $owner | cut -d : -f 1)
> >> -      if [ -n "$pid" -a "$pid" != "unknown" -a ! -f "/proc/$pid/status" ]
> >> -      then
> >> -        _release_lock $lockdir
> >> -      fi
> >> -    fi
> >> -
> >> -    if [ $retries -gt $LOCK_SPINNING_RETRIES ]
> >> -    then
> >> -      sleep $LOCK_SLEEPTIME
> >> -    else
> >> -      sleep 0
> >> -    fi
> >> -    retries=$(($retries + 1))
> >> -  done
> >> -  _steal_lock "$lockdir"
> >> -}
> >> -
> >> -
> >> -_release_lock()
> >> -{
> >> -  trap sigerr ERR
> >> -  rm -rf "$1" 2>/dev/null || true
> >> -}
> >> -
> >> -
> >> -_steal_lock()
> >> -{
> >> -  local lockdir="$1"
> >> -  local owner=$(cat "$lockdir/owner" 2>/dev/null || echo "unknown")
> >> -  log err "Forced to steal lock on $lockdir from $owner!"
> >> -  _release_lock "$lockdir"
> >> -  _claim_lock "$lockdir"
> >> -}
> >> -
> >> -
> >> -_lock_owner()
> >> -{
> >> -  cat "$1/owner" 2>/dev/null || echo "unknown"
> >> -}
> >> -
> >> -
> >> -_update_lock_info()
> >> -{
> >> -  echo "$$: $0" >"$1/owner"
> >> -}
> >>
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@xxxxxxxxxxxxx
> >> http://lists.xen.org/xen-devel
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel
> 



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