netdev - Re: [PATCH net] sctp: add wait_buf flag in asoc to avoid the peeloff and wait sndbuf race

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Date:   Mon, 13 Nov 2017 10:23:00 -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@...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 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

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.

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