Messages in this thread | | | Date | Sat, 16 Apr 2005 13:06:39 +0200 | From | Thomas Graf <> | Subject | Re: ACPI/HT or Packet Scheduler BUG? |
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* Herbert Xu <20050416014906.GA3291@gondor.apana.org.au> 2005-04-16 11:49 > On Fri, Apr 15, 2005 at 10:54:22PM +0000, Thomas Graf wrote: > > > > Another case were it's not locked is upon a deletion of a class where > > the class deletes its inner qdisc, although there is only one case > > how this can happen and that's under rtnl semaphore so there is no > > way we can have a dumper at the same time. > > Sorry, that's where tc went astray :) > > The assumption that the rtnl is held during dumping is false. It is > only true for the initial dump call. All subsequent dumps are not > protected by the rtnl.
Ahh.. it's the unlocked subsequent dump calls that are _still_ running when the destroy is invoked. That's where Patrick and I went wrong when we tried to fix this issue. We set our t0 to qdisc_destroy and didn't really consider any prior unlocked tasks still running.
> In fact the whole qdisc locking is a mess. It's a cross-breed > between locking with ad-hoc reference counting and RCU. What's > more, the RCU is completely useless too because for the case > where we actually have a queue we still end up taking the spin > lock on each transmit! I think someone's been benchmarking the > loopback device again :)
It's not completely useless, it speeds up the deletion classful qdiscs having some depth. However, it's not worth the locking troubles I guess.
> Here is a quick'n'dirty fix to the problem at hand.
I think it's pretty clean but it doesn't solve the problem completely, see below.
> This patch tries to ensure that all top-level calls to qdisc_destroy > come under the tree lock. As Thomas correctedly pointed out, most > of the other qdisc_destroy calls occur after the top qdisc has been > unlinked from the device qdisc_list. However, someone should go > through each one of the remaining ones (they're all in the individual > sch_* implementations) and make sure that this assumption is really > true.
qdisc_destroy can still be invoked without qdisc_tree_lock via the deletion of a class when it calls qdisc_destroy to destroy its leaf qdisc.
> If anyone has cycles to spare and a stomach strong enough for > this stuff, here is your chance :)
I will look into this. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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