Re: [PATCH] Repair misuse of sv_lock in 5.10.16-rt30.
From: J. Bruce Fields
Date: Fri Feb 26 2021 - 10:03:06 EST
Adding Chuck, linux-nfs.
Makes sense to me.--b.
On Fri, Feb 26, 2021 at 09:38:20AM -0500, Joe Korty wrote:
> Repair misuse of sv_lock in 5.10.16-rt30.
>
> [ This problem is in mainline, but only rt has the chops to be
> able to detect it. ]
>
> Lockdep reports a circular lock dependency between serv->sv_lock and
> softirq_ctl.lock on system shutdown, when using a kernel built with
> CONFIG_PREEMPT_RT=y, and a nfs mount exists.
>
> This is due to the definition of spin_lock_bh on rt:
>
> local_bh_disable();
> rt_spin_lock(lock);
>
> which forces a softirq_ctl.lock -> serv->sv_lock dependency. This is
> not a problem as long as _every_ lock of serv->sv_lock is a:
>
> spin_lock_bh(&serv->sv_lock);
>
> but there is one of the form:
>
> spin_lock(&serv->sv_lock);
>
> This is what is causing the circular dependency splat. The spin_lock()
> grabs the lock without first grabbing softirq_ctl.lock via local_bh_disable.
> If later on in the critical region, someone does a local_bh_disable, we
> get a serv->sv_lock -> softirq_ctrl.lock dependency established. Deadlock.
>
> Fix is to make serv->sv_lock be locked with spin_lock_bh everywhere, no
> exceptions.
>
> Signed-off-by: Joe Korty <joe.korty@xxxxxxxxxxxxxxxxx>
>
>
>
>
> [ OK ] Stopped target NFS client services.
> Stopping Logout off all iSCSI sessions on shutdown...
> Stopping NFS server and services...
> [ 109.442380]
> [ 109.442385] ======================================================
> [ 109.442386] WARNING: possible circular locking dependency detected
> [ 109.442387] 5.10.16-rt30 #1 Not tainted
> [ 109.442389] ------------------------------------------------------
> [ 109.442390] nfsd/1032 is trying to acquire lock:
> [ 109.442392] ffff994237617f60 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip+0xd9/0x270
> [ 109.442405]
> [ 109.442405] but task is already holding lock:
> [ 109.442406] ffff994245cb00b0 (&serv->sv_lock){+.+.}-{0:0}, at: svc_close_list+0x1f/0x90
> [ 109.442415]
> [ 109.442415] which lock already depends on the new lock.
> [ 109.442415]
> [ 109.442416]
> [ 109.442416] the existing dependency chain (in reverse order) is:
> [ 109.442417]
> [ 109.442417] -> #1 (&serv->sv_lock){+.+.}-{0:0}:
> [ 109.442421] rt_spin_lock+0x2b/0xc0
> [ 109.442428] svc_add_new_perm_xprt+0x42/0xa0
> [ 109.442430] svc_addsock+0x135/0x220
> [ 109.442434] write_ports+0x4b3/0x620
> [ 109.442438] nfsctl_transaction_write+0x45/0x80
> [ 109.442440] vfs_write+0xff/0x420
> [ 109.442444] ksys_write+0x4f/0xc0
> [ 109.442446] do_syscall_64+0x33/0x40
> [ 109.442450] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 109.442454]
> [ 109.442454] -> #0 ((softirq_ctrl.lock).lock){+.+.}-{2:2}:
> [ 109.442457] __lock_acquire+0x1264/0x20b0
> [ 109.442463] lock_acquire+0xc2/0x400
> [ 109.442466] rt_spin_lock+0x2b/0xc0
> [ 109.442469] __local_bh_disable_ip+0xd9/0x270
> [ 109.442471] svc_xprt_do_enqueue+0xc0/0x4d0
> [ 109.442474] svc_close_list+0x60/0x90
> [ 109.442476] svc_close_net+0x49/0x1a0
> [ 109.442478] svc_shutdown_net+0x12/0x40
> [ 109.442480] nfsd_destroy+0xc5/0x180
> [ 109.442482] nfsd+0x1bc/0x270
> [ 109.442483] kthread+0x194/0x1b0
> [ 109.442487] ret_from_fork+0x22/0x30
> [ 109.442492]
> [ 109.442492] other info that might help us debug this:
> [ 109.442492]
> [ 109.442493] Possible unsafe locking scenario:
> [ 109.442493]
> [ 109.442493] CPU0 CPU1
> [ 109.442494] ---- ----
> [ 109.442495] lock(&serv->sv_lock);
> [ 109.442496] lock((softirq_ctrl.lock).lock);
> [ 109.442498] lock(&serv->sv_lock);
> [ 109.442499] lock((softirq_ctrl.lock).lock);
> [ 109.442501]
> [ 109.442501] *** DEADLOCK ***
> [ 109.442501]
> [ 109.442501] 3 locks held by nfsd/1032:
> [ 109.442503] #0: ffffffff93b49258 (nfsd_mutex){+.+.}-{3:3}, at: nfsd+0x19a/0x270
> [ 109.442508] #1: ffff994245cb00b0 (&serv->sv_lock){+.+.}-{0:0}, at: svc_close_list+0x1f/0x90
> [ 109.442512] #2: ffffffff93a81b20 (rcu_read_lock){....}-{1:2}, at: rt_spin_lock+0x5/0xc0
> [ 109.442518]
> [ 109.442518] stack backtrace:
> [ 109.442519] CPU: 0 PID: 1032 Comm: nfsd Not tainted 5.10.16-rt30 #1
> [ 109.442522] Hardware name: Supermicro X9DRL-3F/iF/X9DRL-3F/iF, BIOS 3.2 09/22/2015
> [ 109.442524] Call Trace:
> [ 109.442527] dump_stack+0x77/0x97
> [ 109.442533] check_noncircular+0xdc/0xf0
> [ 109.442546] __lock_acquire+0x1264/0x20b0
> [ 109.442553] lock_acquire+0xc2/0x400
> [ 109.442564] rt_spin_lock+0x2b/0xc0
> [ 109.442570] __local_bh_disable_ip+0xd9/0x270
> [ 109.442573] svc_xprt_do_enqueue+0xc0/0x4d0
> [ 109.442577] svc_close_list+0x60/0x90
> [ 109.442581] svc_close_net+0x49/0x1a0
> [ 109.442585] svc_shutdown_net+0x12/0x40
> [ 109.442588] nfsd_destroy+0xc5/0x180
> [ 109.442590] nfsd+0x1bc/0x270
> [ 109.442595] kthread+0x194/0x1b0
> [ 109.442600] ret_from_fork+0x22/0x30
> [ 109.518225] nfsd: last server has exited, flushing export cache
> [ OK ] Stopped NFSv4 ID-name mapping service.
> [ OK ] Stopped GSSAPI Proxy Daemon.
> [ OK ] Stopped NFS Mount Daemon.
> [ OK ] Stopped NFS status monitor for NFSv2/3 locking..
> Index: b/net/sunrpc/svc_xprt.c
> ===================================================================
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -1062,7 +1062,7 @@ static int svc_close_list(struct svc_ser
> struct svc_xprt *xprt;
> int ret = 0;
>
> - spin_lock(&serv->sv_lock);
> + spin_lock_bh(&serv->sv_lock);
> list_for_each_entry(xprt, xprt_list, xpt_list) {
> if (xprt->xpt_net != net)
> continue;
> @@ -1070,7 +1070,7 @@ static int svc_close_list(struct svc_ser
> set_bit(XPT_CLOSE, &xprt->xpt_flags);
> svc_xprt_enqueue(xprt);
> }
> - spin_unlock(&serv->sv_lock);
> + spin_unlock_bh(&serv->sv_lock);
> return ret;
> }
>
>