Regarding if_alloc()

[vijay singh]

----- Original Message ----
From: Robert Watson <rwatson at FreeBSD.org>
To: Brooks Davis <brooks at freebsd.org>
Cc: vijay singh <vijjus at rocketmail.com>; freebsd-net at freebsd.org
Sent: Sunday, April 20, 2008 2:27:15 AM
Subject: Re: Regarding if_alloc()


On Fri, 18 Apr 2008, Brooks Davis wrote:

> On Thu, Apr 17, 2008 at 06:35:23PM -0700, vijay singh wrote:
>> Hi all. How do we avoid a race in populating the ifindex_table? Id this is 
>> a TODO, as it seems from the code below, would it be acceptable if I wrote 
>> a patch and reused the ifnet_lock [IFNET_WLOCK, IFNET_WUNLOCK]?
>
> Locking if_index management with ifnet_lock should be ok.  Ideally we should 
> probably be using ALLOC_UNR(9) to manage if_indexes instead of this rather 
> expensive loop.
>
> Be aware, that if_index generation is least of the issues in this area. The 
> if_grow() call is much riskier since it changes the value of the global 
> ifnet pointer which I'm not sure we can afford to lock.  It would be worth 
> experimenting with rmlocks to see what the impact if of locking would be.
>
> I'm serious tempted to kill if_grow in favor of some sort of if_index_max 
> tunable.

I've seen a number of reports of panics that may well be traceable to races in 
if_index handling, and have looked a bit at possible fixes.  Quite a few uses 
of if_index are inherently racy, as they rely on stability of the index value, 
which of course can't be guaranteed with removable interfaces.

I think a reasonable interim fix would be to protect all use of the byindex 
arrays using the ifnet lock, but remember that the read methods, not just the 
write methods, need protection, and as such should move from being macros in 
if_var.h to functions in if.c.  if_grow is probably OK if this is done right, 
but it will need to be set up to drop the lock, grow, re-aquire, and 
re-validate assumptions (i.e., repeat the search for a free index and loop if 
it fails to find one).  Once the read methods are using the lock also, we 
should seriously consider converting it to an rwlock.  We can probably also 
un-publicize at least one of the byindex lookup routines (the dev lookup, 
which is needed only in if.c).

This would prevent races on modifying and evaluating the index array, but not 
on disappearing cdevs and ifnets, which are a separate problem, and one that 
probably is exercised significanty less.  The reports I've seen appear to have 
only to do with pulling the array out from other consumers while in use.


>>

Robert, I am working on the patch, but I had a few questions. If we drop the lock while if_grow() is running, how do we prevent a second caller, blocked in if_alloc() to start scanning the ifindex_table, and end up deciding to grow it again. In other words, we need to stall the ifindex_table scan in if_alloc() till if_grow() has achieved finished. Since if_grow() will be called relatively infrequently, should we consider holding the lock through the routine? Your comments and suggestions are welcome.

-vijay



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