Date: Tue, 16 Aug 2005 20:20:17 -0700 From: Julian Elischer <julian@elischer.org> To: Max Laier <max@love2party.net> Cc: net@freebsd.org, arch@freebsd.org Subject: Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c Message-ID: <4302ACF1.6050209@elischer.org> In-Reply-To: <200508170435.34688.max@love2party.net> References: <20050816170519.A74422@xorpc.icir.org> <200508170435.34688.max@love2party.net>
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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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