From: Joe Korty joe.korty@concurrent-rt.com
commit c7de87ff9dac5f396f62d584f3908f80ddc0e07b upstream.
[ 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.
[ 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..
Fixes: 719f8bcc883e ("svcrpc: fix xpt_list traversal locking on shutdown") Signed-off-by: Joe Korty joe.korty@concurrent-rt.com Signed-off-by: Chuck Lever chuck.lever@oracle.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/sunrpc/svc_xprt.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/net/sunrpc/svc_xprt.c +++ b/net/sunrpc/svc_xprt.c @@ -1011,7 +1011,7 @@ 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; @@ -1019,7 +1019,7 @@ 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; }