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Date: Mon, 2 Jun 2014 20:59:21 -0400 Message-ID: Subject: Re: nfs4_do_reclaim lockdep pop in v3.15.0-rc1 From: Trond Myklebust To: John Stultz Cc: Jeff Layton , Linus Torvalds , Linux NFS Mailing List , Linux Kernel mailing list , Ingo Molnar , Peter Zijlstra Content-Type: text/plain; charset=UTF-8 Sender: linux-nfs-owner@vger.kernel.org List-ID: On Mon, Jun 2, 2014 at 6:49 PM, John Stultz wrote: > On Mon, Jun 2, 2014 at 3:42 PM, Trond Myklebust > wrote: >> On Mon, Jun 2, 2014 at 6:12 PM, John Stultz wrote: >>> On Mon, Jun 2, 2014 at 9:02 AM, Trond Myklebust >>> wrote: >>>> On Mon, Jun 2, 2014 at 10:49 AM, Jeff Layton >>>> wrote: >>>>> I've been working on the patchset to break up the client_mutex in nfsd. >>>>> While doing some debugging, I had mounted my kernel git tree with >>>>> NFSv4.1, and was running crash on the vmlinux image in it. >>>>> >>>>> A little while later, I saw the following lockdep inversion pop. >>>>> Unfortunately, I couldn't get the whole log, but I think it's enough to >>>>> show that there's a potential problem? >>>>> >>>>> I've not had time to give it a hard look yet, but thought I'd post it >>>>> here in the hopes that it might look familiar to someone: >>>>> >>>>> [ 2581.104687] ====================================================== >>>>> [ 2581.104716] [ INFO: possible circular locking dependency detected ] >>>>> [ 2581.104716] 3.15.0-rc1.jlayton.1+ #2 Tainted: G OE >>>>> [ 2581.104716] ------------------------------------------------------- >>>>> [ 2581.104716] 2001:470:8:d63:/5622 is trying to acquire lock: >>>>> [ 2581.104716] (&(&sp->so_lock)->rlock){+.+...}, at: [] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4] >>>>> [ 2581.104716] >>>>> [ 2581.104716] but task is already holding lock: >>>>> [ 2581.104716] (&sp->so_reclaim_seqcount){+.+...}, at: [] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] >>>>> [ 2581.104716] which lock already depends on the new lock. >>>>> [ 2581.104716] >>>>> [ 2581.104716] >>>>> [ 2581.104716] the existing dependency chain (in reverse order) is: >>>>> [ 2581.104716] >>>>> -> #1 (&sp->so_reclaim_seqcount){+.+...}: >>>>> [ 2581.104716] [] lock_acquire+0xa2/0x1d0 >>>>> [ 2581.104716] [] nfs4_do_reclaim+0x290/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] [] kthread+0xff/0x120 >>>>> [ 2581.104716] [] ret_from_fork+0x7c/0xb0 >>>>> [ 2581.104716] >>>>> -> #0 (&(&sp->so_lock)->rlock){+.+...}: >>>>> [ 2581.104716] [] __lock_acquire+0x1b8f/0x1ca0 >>>>> [ 2581.104716] [] lock_acquire+0xa2/0x1d0 >>>>> [ 2581.104716] [] _raw_spin_lock+0x3e/0x80 >>>>> [ 2581.104716] [] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] [] kthread+0xff/0x120 >>>>> [ 2581.104716] [] ret_from_fork+0x7c/0xb0 >>>>> [ 2581.104716] >>>>> [ 2581.104716] other info that might help us debug this: >>>>> [ 2581.104716] >>>>> [ 2581.104716] Possible unsafe locking scenario: >>>>> [ 2581.104716] >>>>> [ 2581.104716] CPU0 CPU1 >>>>> [ 2581.104716] ---- ---- >>>>> [ 2581.104716] lock(&sp->so_reclaim_seqcount); >>>>> [ 2581.104716] lock(&(&sp->so_lock)->rlock); >>>>> [ 2581.104716] lock(&sp->so_reclaim_seqcount); >>>>> [ 2581.104716] lock(&(&sp->so_lock)->rlock); >>>>> [ 2581.104716] >>>>> [ 2581.104716] *** DEADLOCK *** >>>>> [ 2581.104716] >>>>> [ 2581.104716] 1 lock held by 2001:470:8:d63:/5622: >>>>> [ 2581.104716] #0: (&sp->so_reclaim_seqcount){+.+...}, at: [] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] >>>>> [ 2581.104716] stack backtrace: >>>>> [ 2581.104716] CPU: 2 PID: 5622 Comm: 2001:470:8:d63: Tainted: G OE 3.15.0-rc1.jlayton.1+ #2 >>>>> [ 2581.104716] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 >>>>> [ 2581.104716] 0000000000000000 00000000d29e16c4 ffff8800d8d8fba8 ffffffff817d318e >>>>> [ 2581.104716] ffffffff8262d5e0 ffff8800d8d8fbe8 ffffffff817ce525 ffff8800d8d8fc40 >>>>> [ 2581.104716] ffff8800362a8b98 ffff8800362a8b98 0000000000000001 ffff8800362a8000 >>>>> [ 2581.104716] Call Trace: >>>>> [ 2581.104716] [] dump_stack+0x4d/0x66 >>>>> [ 2581.104716] [] print_circular_bug+0x201/0x20f >>>>> [ 2581.104716] [] __lock_acquire+0x1b8f/0x1ca0 >>>>> [ 2581.104716] [] ? debug_check_no_obj_freed+0x17e/0x270 >>>>> [ 2581.104716] [] lock_acquire+0xa2/0x1d0 >>>>> [ 2581.104716] [] ? nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] _raw_spin_lock+0x3e/0x80 >>>>> [ 2581.104716] [] ? nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] ? nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] [] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4] >>>>> [ 2581.104716] [] ? nfs4_do_reclaim+0x7f0/0x7f0 [nfsv4] >>>>> [ 2581.104716] [] kthread+0xff/0x120 >>>>> [ 2581.104716] [] ? insert_kthread_work+0x80/0x80 >>>>> [ 2581.104716] [] ret_from_fork+0x7c/0xb0 >>>>> [ 2581.104716] [] ? insert_kthread_work+0x80/0x80 >>>> >>>> OK. So now that lockdep has been added to raw_seqcount_begin() (commit >>>> 1ca7d67cf5d5a), exactly what are we supposed to use when we DON'T want >>>> lockdep to "sanity check" our locking here? >>> >>> So raw_write_seqcount_* provides the lockdep-disabled methods. >>> >>>> As far as we're concerned, the above check is completely bogus, and >>>> there is no deadlock. At best it would be a livelock, and it would be >>>> because the server is rebooting over and over again (in which case the >>>> client behaviour of retrying is _correct_). >>> >>> I've not been able to totally trace the locking path there, but having >>> a seqlock writes and spinlock ABBA deadlock seems problematic.... at >>> least at first glance. >>> >>> So if I'm reading this right... nfs4_reclaim_open_state() takes a >>> spinlock on so_lock, then the write on the so_reclaim_seqcount, then >>> drops the so_lock and calls nfs4_put_open_state which reaquires the >>> so_lock. >>> >>> And lockdep is worried there may be another thread which called into >>> nfs4_reclaim_open_state() and took the so_lock while it was >>> momentarily free, and is blocking waiting on the so_reclaim_seqcount. >>> This would cause the first threads requisition of the so_lock to >>> potentially deadlock. >>> >>> And your point is that this isn't a concern since no other threads can >>> call nfs4_reclaim_open_state() or any other code path that acquires >>> those two locks in order? If you're going to disable the lockdep >>> checks here, you might want to make this restriction really clear in a >>> comment so no one accidentally breaks that rule. >>> >> >> It's a general rule in the NFSv4 client that the only thread that is >> allowed to call state recovery functions (after a server reboot or a >> long-lived network partition) is the state manager thread. That >> applies to _all_ state, not just open state. >> >> The so_reclaim_seqcount only exists in order to tell the other threads >> that they may need to replay file open or file lock requests that have >> raced with state recovery (because those threads got scheduled out >> after their RPC calls ran, but before they managed to set up the >> tracking of the new state). It is basically an edge condition >> killer... > > Would then swapping the acquisition order, so the seqcount is taken > before the so_lock at the top of nfs4_reclaim_open_state() avoid this > then, without having to disable lockdep? > I can change the write seqcount to use raw_write_seqcount(), but that doesn't answer the question of why raw_seqcount_begin() is the _only_ object out there with a "raw_" prefix, that doesn't explicitly disable lockdep checking. What justifies the inconsistency? -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@primarydata.com