Re: [syzbot] [rds?] possible deadlock in rds_tcp_tune (2)

[syzbot <syzbot+2e2cf5331207053b8106@syzkaller.appspotmail.com>]

> #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main

This crash does not have a reproducer. I cannot test it.

>
> commit 4cd6716706210de3ed52d549ee784a12cc8ffe3a (HEAD)
> Author: Allison Henderson <achender@kernel.org>
> Date:   Thu Feb 26 12:45:39 2026 -0700
>
>     net/rds: Fix circular locking dependency in rds_tcp_tune
>     
>     syzbot reported a circular locking dependency in rds_tcp_tune() where
>     sk_net_refcnt_upgrade() is called while holding the socket lock:
>     
>     ======================================================
>     WARNING: possible circular locking dependency detected
>     ------------------------------------------------------
>     kworker/u10:8/15040 is trying to acquire lock:
>     ffffffff8e9aaf80 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc_cache_noprof+0x4b/0x6f0
>     
>     but task is already holding lock:
>     ffff88805a3c1ce0 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: rds_tcp_tune+0xd7/0x930
>     
>     The issue occurs because sk_net_refcnt_upgrade() performs memory allocation
>     (via get_net_track() -> ref_tracker_alloc()) while the socket lock is held,
>     creating a circular dependency with fs_reclaim.
>     
>     Fix this by moving sk_net_refcnt_upgrade() outside the socket lock critical
>     section. Since the fresh socket is not yet exposed to other threads, no
>     locks are needed at this time.
>     
>     Reported-by: syzbot+2e2cf5331207053b8106@syzkaller.appspotmail.com
>     Closes: https://syzkaller.appspot.com/bug?extid=2e2cf5331207053b8106
>     Fixes: 5c70eb5c593d ("net: better track kernel sockets lifetime")
>     Signed-off-by: Allison Henderson <achender@kernel.org>
>
> diff --git a/net/rds/tcp.c b/net/rds/tcp.c
> index 04f310255692..da22b3dfdbf0 100644
> --- a/net/rds/tcp.c
> +++ b/net/rds/tcp.c
> @@ -490,18 +490,24 @@ bool rds_tcp_tune(struct socket *sock)
> commit 4cd6716706210de3ed52d549ee784a12cc8ffe3a (HEAD)
> Author: Allison Henderson <achender@kernel.org>
> Date:   Thu Feb 26 12:45:39 2026 -0700
>
>     net/rds: Fix circular locking dependency in rds_tcp_tune
>     
>     syzbot reported a circular locking dependency in rds_tcp_tune() where
>     sk_net_refcnt_upgrade() is called while holding the socket lock:
>     
>     ======================================================
>     WARNING: possible circular locking dependency detected
>     ------------------------------------------------------
>     kworker/u10:8/15040 is trying to acquire lock:
>     ffffffff8e9aaf80 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc_cache_noprof+0x4b/0x6f0
>     
>     but task is already holding lock:
>     ffff88805a3c1ce0 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: rds_tcp_tune+0xd7/0x930
>     
>     The issue occurs because sk_net_refcnt_upgrade() performs memory allocation
>     (via get_net_track() -> ref_tracker_alloc()) while the socket lock is held,
>     creating a circular dependency with fs_reclaim.
>     
>     Fix this by moving sk_net_refcnt_upgrade() outside the socket lock critical
>     section. Since the fresh socket is not yet exposed to other threads, no
>     locks are needed at this time.
>     
>     Reported-by: syzbot+2e2cf5331207053b8106@syzkaller.appspotmail.com
>     Closes: https://syzkaller.appspot.com/bug?extid=2e2cf5331207053b8106
>     Fixes: 5c70eb5c593d ("net: better track kernel sockets lifetime")
>     Signed-off-by: Allison Henderson <achender@kernel.org>
>
> diff --git a/net/rds/tcp.c b/net/rds/tcp.c
> index 04f310255692..da22b3dfdbf0 100644
> --- a/net/rds/tcp.c
> +++ b/net/rds/tcp.c
> @@ -490,18 +490,24 @@ bool rds_tcp_tune(struct socket *sock)
>         struct rds_tcp_net *rtn;
>  
>         tcp_sock_set_nodelay(sock->sk);
> -       lock_sock(sk);
>         /* TCP timer functions might access net namespace even after
>          * a process which created this net namespace terminated.
>          */
>         if (!sk->sk_net_refcnt) {
> -               if (!maybe_get_net(net)) {
> -                       release_sock(sk);
> +               if (!maybe_get_net(net))
>                         return false;
> -               }
> +               /*
> +                * We call sk_net_refcnt_upgrade before the lock_sock since it is
> +                * not yet shared, no lock is needed at this time.  Further,
> +                * because sk_net_refcnt_upgrade does a GFP_KERNEL allocation,
> +                * this can trigger an fs_reclaim in other systems which creates
> +                * a circular lock dependancy.  Avoid this by upgrading the
> +                * refcnt before the locking the socket.
> +                */
>                 sk_net_refcnt_upgrade(sk);
>                 put_net(net);
>         }
> +       lock_sock(sk);
>         rtn = net_generic(net, rds_tcp_netid);
>         if (rtn->sndbuf_size > 0) {
>                 sk->sk_sndbuf = rtn->sndbuf_size;
> @@ -490,18 +490,24 @@ bool rds_tcp_tune(struct socket *sock)
>         struct rds_tcp_net *rtn;
>  
>         tcp_sock_set_nodelay(sock->sk);
> -       lock_sock(sk);
>         /* TCP timer functions might access net namespace even after
>          * a process which created this net namespace terminated.
>          */
>         if (!sk->sk_net_refcnt) {
> -               if (!maybe_get_net(net)) {
> -                       release_sock(sk);
> +               if (!maybe_get_net(net))
>                         return false;
> -               }
> +               /*
> +                * We call sk_net_refcnt_upgrade before the lock_sock since it is
> +                * not yet shared, no lock is needed at this time.  Further,
> +                * because sk_net_refcnt_upgrade does a GFP_KERNEL allocation,
> +                * this can trigger an fs_reclaim in other systems which creates
> +                * a circular lock dependancy.  Avoid this by upgrading the
> +                * refcnt before the locking the socket.
> +                */
>                 sk_net_refcnt_upgrade(sk);
>                 put_net(net);
>         }
> +       lock_sock(sk);
>         rtn = net_generic(net, rds_tcp_netid);
>         if (rtn->sndbuf_size > 0) {
>                 sk->sk_sndbuf = rtn->sndbuf_size;
>