Date: Tue, 11 Apr 2000 13:40:49 -0700 From: "Wilbert de Graaf" <wilbertdg@hetnet.nl> To: "FreeBSD-hackers" <FreeBSD-hackers@FreeBSD.ORG> Subject: Refresh a list that could be used Message-ID: <007501bfa3f6$556cf6f0$fec92080@alias>
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Hi,
As part of an IGMPv3 implementation on FreeBSD I had to add a list to a =
datastructure (in_multi). This list can be set by users (using ioctl) =
and be replaced later. While replacing this list, there is the =
possibility (ad1) of a concurrent reader (the process when an incoming =
packet has to be passed to a socket). So just replacing this list and =
deleting the old one makes no sense.
There could think of two ways to implement this:
1) a locking strategy: lock when updating and while being used ... =
however, this slows down the process of incoming packets
-or-
2) simply save the old list, and delete it at the next replacement of =
the list (ad 2)
Or is there a common strategy in FreeBSD to handle a situation like this =
? Could anybody comment on this.=20
Thanks, Wilbert
Ad 1)
It is true that this replacement of the list will *not* preempt in this =
case since the reader is delivery of an ip packet to a socket, which is =
done at splnet(), while this routine doesn't even use splimp(). But this =
precaution is just to make sure it will be safe in the case of =
multi-processors. Or did I miss something ?
Ad 2)
struct in_multi {
...
struct isf_entry *inm_sflist, *inm_sfoldlist; /* source =
filter list */
...
};
While replacing the list, the old one is stored in inm_sfoldlist, so =
current readers won't be harmed by invalid pointers while deleting the =
list. At a next replacement, the old list is simply deleted since the =
assumption is really valid that there won't be any readers of the old =
list. (Not eve when a multi-threaded program starts to fool around with =
the filterlist).
Btw. There will be a hashing schema over this structure, to determine =
whether a source ip is in the set.
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<META content=3D"text/html; charset=3Diso-8859-1" =
http-equiv=3DContent-Type>
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<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>Hi,</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>As part of an IGMPv3 implementation on =
FreeBSD I=20
had to add a list to a datastructure (in_multi). This list can be set by =
users=20
(using ioctl) and be replaced later.</FONT><FONT face=3DArial =
size=3D2> While=20
replacing this list, there is the possibility (ad1) of a concurrent =
reader (the=20
process when an incoming packet has to be passed to a socket). So =
just=20
replacing this list and deleting the old one makes no =
sense.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>There could think of two ways to =
implement=20
this:</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>1) a locking strategy: lock when =
updating and while=20
being used ... however, this slows down the process of incoming=20
packets</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>-or-</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>2) simply save the old list, and delete =
it at the=20
next replacement of the list (ad 2)</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Or is there a common strategy in =
FreeBSD to handle=20
a situation like this ? Could anybody comment on this. =
</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Thanks, Wilbert</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Ad 1)</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>
<DIV><FONT face=3DArial size=3D2>It is true that this replacement =
of the=20
list will *not* preempt in this case since the reader =
is delivery of=20
an ip packet to a socket, which is done at splnet(), while this =
routine=20
doesn't even use splimp(). But this precaution is just to make sure it =
will be=20
safe in the case of multi-processors. Or did I miss something =
?</FONT></DIV>
<DIV> </DIV>Ad 2)</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>struct in_multi {</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> ...</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> struct isf_entry =
*inm_sflist,=20
*inm_sfoldlist; /* source filter =
list=20
*/</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> ...</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>};</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>While replacing the list, the old one =
is stored in=20
inm_sfoldlist, so current readers won't be harmed by invalid pointers =
while=20
deleting the list. At a next replacement, the old list is simply deleted =
since=20
the assumption is really valid that there won't be any readers of the =
old list.=20
(Not eve when a multi-threaded program starts to fool around with the=20
filterlist).</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Btw. There will be a hashing schema =
over this=20
structure, to determine whether a source ip is in the set.</FONT></DIV>
<DIV> </DIV></BODY></HTML>
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