Re: Possible ABA in use of llist.h llist_del_first() in tty_buffer and ib_rdma

[Mathieu Desnoyers]

CCing LKML.

----- On Mar 31, 2016, at 5:39 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:

> Hi,
> 
> Code review (really: grepping the Linux kernel for
> llist_del_first) leads me to notice two possible ABA issues.
> The first one is in drivers/tty/tty_buffer.c, and is due to
> its use of llist_del_all and llist_del_first without locking
> since commit 809850b7a5 "tty: Use lockless flip buffer free list".
> 
> Unfortunately, it appears to do so without respecting the
> locking requirements associated with llist_del_first.
> 
> Quoting llist.h:
> 
> " * If there are multiple producers and one consumer, llist_add can be
> * used in producers and llist_del_all or llist_del_first can be used
> * in the consumer.
> *
> * This can be summarized as follow:
> *
> *           |   add    | del_first |  del_all
> * add       |    -     |     -     |     -
> * del_first |          |     L     |     L
> * del_all   |          |           |     -
> *
> * Where "-" stands for no lock is needed, while "L" stands for lock
> * is needed.
> "
> 
> As soon as a llist_del_first() is used, then both llist_del_first()
> and llist_del_all() need to be protected by a lock, thus preventing
> ABA in llist_del_first().
> 
> An alternative to locking would be to only use llist_del_all() and
> never llist_del_first().
> 
> I'm also noticing a similar concurrent use of llist_del_first() and
> llist_del_all() in commit 1bc144b625 "net, rds, Replace xlist in net/rds/xlist.h
> with llist".
> The locking surrounding their use (especially in rds_ib_reuse_mr)
> don't appear clearly documented there. Perhaps there was a preexisting
> issue with the xlist.h use too ?
> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com