Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c
From: Julian Elischer (julian_at_elischer.org)
Date: 08/17/05
- Previous message: Max Laier: "Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c"
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Date: Tue, 16 Aug 2005 20:20:17 -0700 To: Max Laier <max@love2party.net>
Max Laier wrote:
> On Wednesday 17 August 2005 02:05, Luigi Rizzo wrote:
>
>>[apologies for the cross post but it belongs both to arch and net.]
>>
>>I notice that net/pfil.c and netinet/ip_fw2.c have two copies of
>>aisimilar but slightly different implementation of
>>multiple-reader/single-writer locks, which brings up the question(s):
>>
>>1. should we rather put this code in the generic kernel code so that other
>> subsystems could make use of it ? E.g. the routing table is certainly
>> a candidate,
>
>
> I have asked this several time on -arch and IRC, but never found anyone
> willing to pursue it. However, the problem is ...
>
>
>>and especially
>>
>>2. should we implement it right ?
>>
>> Both implementations are subject to starvation for the writers
>> (which is indeed a problem here, because we might want to modify
>> a ruleset and be prevented from doing it because of incoming traffic
>> that keeps readers active).
>> Also the PFIL_TRY_WLOCK will in fact be blocking if a writer
>> is already in - i have no idea how problematic is this in the
>> way it is actually used.
>
>
> ... really this. I didn't find a clean way out of the starvation issue. What
> I do for pfil is that I set a flag and simply stop serving[2] shared requests
> once a writer waits for the lock. If a writer can't sleep[1] then we return
> EBUSY and don't. However, for pfil it's almost ever safe to assume that a
> write may sleep (as it is for most instances of this kind of sx-lock where
> you have BIGNUMxreads:1xwrite).
>
> [1] Note that there is a *big* difference between blocking and sleeping.
> These two are usually confused. While it is almost always okay to block it
> is seldom okay to sleep. The existing sx(9) api has the problem that it
> *sleeps* in the shared path which renders it unusable for this usecase (as we
> might be holding other locks and must not sleep in the shared path).
> However, sleeping in the shared path is one (?the only?) way out of the
> starvation problem - other than a problem specific as done for pfil.
>
> [2] See pfil(9) BUGS.
netgraph has yet another implementation of R/W locks.
It relies on the fact that every lock action is done on behalf of
a command request or a data processing request, each of which is
queueable, and each RW lock is associated with a queue.
Instead of blocking, the item is queued instead for later processing.
>
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- Previous message: Max Laier: "Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c"
- In reply to: Max Laier: "Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c"
- Next in thread: Luigi Rizzo: "Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c"
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