Re: [GIT PULL] kdbus for 4.1-rc1

From: Greg Kroah-Hartman
Date: Wed Apr 15 2015 - 09:13:22 EST

On Wed, Apr 15, 2015 at 01:36:33PM +0100, Al Viro wrote:
> al@duke:~/linux/trees/vfs$ git grep -n -w kdbus_node_idr_lock
> ipc/kdbus/node.c:237:static DECLARE_RWSEM(kdbus_node_idr_lock);
> ipc/kdbus/node.c:340: down_write(&kdbus_node_idr_lock);
> ipc/kdbus/node.c:344: up_write(&kdbus_node_idr_lock);
> ipc/kdbus/node.c:444: down_write(&kdbus_node_idr_lock);
> ipc/kdbus/node.c:452: up_write(&kdbus_node_idr_lock);

Heh, that's a leftover from an older version, I'll go fix that up to be
a simple mutex, which is all that this is doing here anyway.

> Do you see anything wrong with that? Or with things like that:
> mutex_lock(&pos->lock);
> v_pre = atomic_read(&pos->active);
> if (v_pre >= 0)
> atomic_add_return(KDBUS_NODE_BIAS, &pos->active);
> else if (v_pre == KDBUS_NODE_NEW)
> atomic_set(&pos->active, KDBUS_NODE_RELEASE_DIRECT);
> mutex_unlock(&pos->lock);
> What are the locking rules for ->active/->waitq/->lock? Are those the
> outermost thing in the hierarchy? Or is that dependent on the node location?
> It sure as hell is outside of (at least) ->mmap_sem (by way of
> kdbus_conn_connect() establishing that ->active/->waitq is outside of
> ->conn_rwlock, which due to kdbus_bus_broadcast() nests outside of anything
> taken by kdbus_meta_proc_collect(), which includes ->mmap_sem) and that alone
> brings in a lot...
> Document your goddamn locking, would you? It *IS* new code, and you, as you
> say, had very few people working on it, so you don't have the excuses for
> the mess existing in older parts of the tree.

Fair enough, documenting the locking is a good thing, that will make
reviewing this easier, I'll go work on that.

> Locking complexity in there is easily as bad as that of VFS sans the RCU fun;
> sure, I can spend a week and (hopefully) document it for you, but I would
> really prefer if you guys had done that. And I *do* appreciate the comments
> in node.c, but they are nowhere near enough.

Thanks, it's hard to balance the comment/code level at times. And yes,
it is complex and should be explained better, will work on that.

greg k-h
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