Re: [PATCH net v3 04/11] list: Move on_list_rcu() to list.h and add on_list() also

[David Howells <dhowells@redhat.com>]

Linus Torvalds <torvalds@linux-foundation.org> wrote:

> Dammit, you should *KNOW* that already from core logic. Not with a
> flag, not with a function to ask, but from how things work. The whole
> "am I on a list or not" should not be a list issue, it should be
> obvious.

The circumstance in question is this: There's a list of outstanding calls
attached to the rxrpc network namespace.  Calls may hang around on it beyond
the life of the socket that created them for a little bit to deal with network
protocol cleanup, timer cleanup, work func cleanup.  Under normal operation,
calls are removed as the last ref is put.

However, should the namespace be deleted, rxrpc_destroy_all_calls() trawls the
list to report any calls that haven't been cleaned up and the calls are
deleted from the list as it reports them so that the spinlock doesn't have to
be kept held.  It used to do other work here too, IIRC, but that's no longer
the case, so perhaps this loop is not needed now, and a warning will suffice
if the list is not empty (or I could just report, say, the first 10 calls and
not worry about calling cond_resched()).  The wait at the bottom of the
function should be sufficient to hold up namespace deallocation.

If I don't delete entries in rxrpc_destroy_all_calls(), then rxrpc_put_call()
only needs list_empty() to guard against the call not having being queued yet.
I could have a flag for that, but it would be superfluous.

Note that the reason for the RCU walking is because /proc/net/rxrpc/calls
walks over the same list, so I still need the next patch which switches to
list_del_rcu().

David