Subject: Re: Deadlock in IPv6 code while garbage collection on the rwlock protecting the routing tree.

From: Stephen Hemminger <[email-masked]>
Date: Tue, 22 Dec 2009 14:36:38 -0800

On Tue, 22 Dec 2009 16:57:05 -0500
"Akkipeddi, Srinivas" <[email-masked]> wrote:

> I came across a deadlock scenario in the latest IPv6 code. I am trying
> to fix this and any inputs are really appreciated. 
> 
> The deadlock happens when ROUTER-PREF is configured. This happens when
> trying to do a write_lock_bh on the rwlock protecting the routing tree
> during garbage collection.
> 
> The routing tree is read protected (read_lock_bh(&table->tb6_lock))
> using the rwlock when performing a ip6_route_input or  ip6_route_output
> ( "ip6_pol_route"). During route selection (rt6_select), if a neighbor
> solicit is sent (ndisc_send_ns), a dst_entry is allocated
> (icmp6_dst_alloc calls dst_alloc). 
> The garbage collection (fib6_run_gc) will be triggered if the number of
> dst-entries is more than the threshold (dst_alloc). During garbage
> collection, all the routing trees are cleaned up (fib6_clean_all). Here
> we try to take write protect each routing tree (
> write_lock_bh(&table->tb6_lock)). But one of the trees is already read
> protected. 
> 
> The garbage collection is anyways triggered from "icmp6_dst_alloc" with
> the call to fib6_force_start_gc. Since it is triggered, we might not
> want to call the "fib6_run_gc" from dst_alloc for this case but there is
> no way to figure this out in the "dst_alloc" routine.

Might just be easier to convert to spinlock and RCU.

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