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Date:   Tue, 14 Nov 2017 09:31:58 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem <davem@...emloft.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net] sctp: add wait_buf flag in asoc to avoid the peeloff
 and wait sndbuf race

On Mon, Nov 13, 2017 at 11:49:28PM +0800, Xin Long wrote:
> On Mon, Nov 13, 2017 at 11:40 PM, Xin Long <lucien.xin@...il.com> wrote:
> > On Mon, Nov 13, 2017 at 11:23 PM, Neil Horman <nhorman@...driver.com> wrote:
> >> On Mon, Nov 13, 2017 at 01:47:58PM +0800, Xin Long wrote:
> >>> Commit dfcb9f4f99f1 ("sctp: deny peeloff operation on asocs with threads
> >>> sleeping on it") fixed the race between peeloff and wait sndbuf by
> >>> checking waitqueue_active(&asoc->wait) in sctp_do_peeloff().
> >>>
> >>> But it actually doesn't work as even if waitqueue_active returns false
> >>> the waiting sndbuf thread may still not yet hold sk lock.
> >>>
> >>> This patch is to fix this by adding wait_buf flag in asoc, and setting it
> >>> before going the waiting loop, clearing it until the waiting loop breaks,
> >>> and checking it in sctp_do_peeloff instead.
> >>>
> >>> Fixes: dfcb9f4f99f1 ("sctp: deny peeloff operation on asocs with threads sleeping on it")
> >>> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> >>> Signed-off-by: Xin Long <lucien.xin@...il.com>
> >>> ---
> >>>  include/net/sctp/structs.h | 1 +
> >>>  net/sctp/socket.c          | 4 +++-
> >>>  2 files changed, 4 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
> >>> index 0477945..446350e 100644
> >>> --- a/include/net/sctp/structs.h
> >>> +++ b/include/net/sctp/structs.h
> >>> @@ -1883,6 +1883,7 @@ struct sctp_association {
> >>>
> >>>       __u8 need_ecne:1,       /* Need to send an ECNE Chunk? */
> >>>            temp:1,            /* Is it a temporary association? */
> >>> +          wait_buf:1,
> >>>            force_delay:1,
> >>>            prsctp_enable:1,
> >>>            reconf_enable:1;
> >>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> >>> index 6f45d17..1b2c78c 100644
> >>> --- a/net/sctp/socket.c
> >>> +++ b/net/sctp/socket.c
> >>> @@ -4946,7 +4946,7 @@ int sctp_do_peeloff(struct sock *sk, sctp_assoc_t id, struct socket **sockp)
> >>>       /* If there is a thread waiting on more sndbuf space for
> >>>        * sending on this asoc, it cannot be peeled.
> >>>        */
> >>> -     if (waitqueue_active(&asoc->wait))
> >>> +     if (asoc->wait_buf)
> >>>               return -EBUSY;
> >>>
> >>>       /* An association cannot be branched off from an already peeled-off
> >>> @@ -7835,6 +7835,7 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >>>       /* Increment the association's refcnt.  */
> >>>       sctp_association_hold(asoc);
> >>>
> >>> +     asoc->wait_buf = 1;
> >>>       /* Wait on the association specific sndbuf space. */
> >>>       for (;;) {
> >>>               prepare_to_wait_exclusive(&asoc->wait, &wait,
> >>> @@ -7860,6 +7861,7 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >>>       }
> >>>
> >>>  out:
> >>> +     asoc->wait_buf = 0;
> >>>       finish_wait(&asoc->wait, &wait);
> >>>
> >>>       /* Release the association's refcnt.  */
> >>> --
> >>> 2.1.0
> >>>
> >>>
> >>
> >> This doesn't make much sense to me, as it appears to be prone to aliasing.  That
> >> is to say:
> >>
> >> a) If multiple tasks are queued waiting in sctp_wait_for_sndbuf, the first
> >> thread to exit that for(;;) loop will clean asoc->wait_buf, even though others
> >> may be waiting on it, allowing sctp_do_peeloff to continue when it shouldn't be
> > You're right, we talked about this before using waitqueue_active in
> > earlier time.
> > I didn't remember this somehow. Sorry.
> >
> >>
> >> b) In the case of a single task blocking in sct_wait_for_sendbuf, checking
> >> waitqueue_active is equally good, because it returns true, until such time as
> >> finish_wait is called anyway.
> > waitqueue_active can not work here, because in sctp_wait_for_sndbuf():
> > ...
> >                 release_sock(sk);
> >                 current_timeo = schedule_timeout(current_timeo); <-----[a]
> >                 lock_sock(sk);
> > If another thread wakes up asoc->wait, it will be removed from
> > this wait queue, you check DEFINE_WAIT, the callback autoremove_wake_function
> > will do this removal in wake_up().
> >
> > I guess we need to think about another to fix this.
> maybe we can use
> DEFINE_WAIT_FUNC(wait, woken_wake_function);
> instead of DEFINE_WAIT(wait) here ?
> 
I'm still not sure I see the problem here.  If we have the following situation:
* Exec context A is executing in sctp_do_peeloff, about to check
  waitqueue_active() 
* Exec context B is blocking in sctp_wait_for_sndbuf(), specifically without the
  socket lock held


Then, we have two possibilities:

a) Context A executes waitqueue_active, which returns true, implying that
context B is still on the queue, or that some other undescribed context has
begun waiting on the queue.  In either case, the behavior is correct, in that
the peeloff is denied.

b) Context B is woken up (and in the most pessimal case, has its waitq entry
removed from queue immediately, causing context B to have waitequeue_active
return false, allowing it to continue processing the peeloff.  Since it holds
the socket lock however, context B will block on the lock_sock operation until
such time as the peeloff completes, so you're safe.

About the only issue that I see (and as I write this, I may be seeing what you
are actually trying to fix here) is that, during the period where context A is
sleeping in sctp_wait_for_sendbuf, with the socket lock released, it is possible
for an sctp_do_peeloff operation to complete, meaning that assoc->base.sk might
point to a new socket, allowing each context to hold an independent socket lock
and execute in parallel.  To combat that, I think all you really need is some
code in sctp_wait_for_sndbuf that looks like this:

release_sock(sk);
current_timeo = schedule_timeout(current_timeo);
lock_sock(sk);

if (sk != asoc->base.sk) {
	/* a socket peeloff occured */
	release_sock(sk);
	sk = assoc->base.sk;
	lock_sock(sk);
}

*timeo_p = current_timeo;


Does that make sense?  This way, you lock the 'old' socket lock to ensure that
the peeloff operation is completed, then you check to see if the socket has
changed.  If it has, you migrate your socket to the new, peeled off one and
continue your space availability check

Neil


Neil




 >
> >>
> >> It really seems to me that waitqueue_active is the right answer here, as it
> >> should return true until there are no longer any tasks waiting on sndbuf space
> >>
> >> Neil
> >>
> 

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