Date:	Tue, 3 Feb 2009 14:03:13 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	MinChan Kim <minchan.kim@...il.com>, linux-fsdevel@...r.kernel.org,
	john@...nmccutchan.com, rlove@...ve.org
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	linux mm <linux-mm@...ck.org>
Subject: Re: [BUG??] Deadlock between kswapd and sys_inotify_add_watch(lockdep report)

Let's add fsdevel.

On Monday 02 February 2009 21:17:35 MinChan Kim wrote:
> Nick's new lockdep of 'annotate reclaim context(__GFP_NOFS)' reported
> following message. In my kernel(2.6.28-rc2-mm1 + nick's patch :
> http://patchwork.kernel.org/patch/4251/),
>
> During 'dd if=/dev/zero of=test.image bs=4096 count=2621440' of two
> processes, following message occured.
>
> I think it might be useful case of 'annotate reclaim context'.
>
> [  331.718116] =================================
> [  331.718120] [ INFO: inconsistent lock state ]
> [  331.718124] 2.6.28-rc2-mm1-lockdep #6
> [  331.718126] ---------------------------------
> [  331.718129] inconsistent {ov-reclaim-W} -> {in-reclaim-W} usage.
> [  331.718133] kswapd0/218 [HC0[0]:SC0[0]:HE0:SE1] takes:
> [  331.718136]  (&inode->inotify_mutex){--..+.}, at: [<c01dba70>]
> inotify_inode_is_dead+0x20/0x90 [  331.718148] {ov-reclaim-W} state was
> registered at:
> [  331.718150]   [<c01532ee>] mark_held_locks+0x3e/0x90
> [  331.718157]   [<c015338e>] lockdep_trace_alloc+0x4e/0x80
> [  331.718162]   [<c01acee6>] kmem_cache_alloc+0x26/0xf0
> [  331.718166]   [<c0243fa0>] idr_pre_get+0x50/0x70
> [  331.718172]   [<c01db761>] inotify_handle_get_wd+0x21/0x60
> [  331.718176]   [<c01dc012>] inotify_add_watch+0x52/0xe0
> [  331.718181]   [<c01dcca8>] sys_inotify_add_watch+0x148/0x170
> [  331.718185]   [<c0104032>] syscall_call+0x7/0xb

So we can enter __GFP_FS reclaim here, with inode->inotify_mutex held.


> [  331.718190]   [<ffffffff>] 0xffffffff
> [  331.718205] irq event stamp: 1288446
> [  331.718207] hardirqs last  enabled at (1288445): [<c0179695>]
> call_rcu+0x75/0x90 [  331.718213] hardirqs last disabled at (1288446):
> [<c0370103>] mutex_lock_nested+0x53/0x2f0 [  331.718221] softirqs last 
> enabled at (1284622): [<c0132fa2>] __do_softirq+0x132/0x180 [  331.718226]
> softirqs last disabled at (1284617): [<c0133079>] do_softirq+0x89/0x90 [ 
> 331.718231]
> [  331.718232] other info that might help us debug this:
> [  331.718236] 2 locks held by kswapd0/218:
> [  331.718238]  #0:  (shrinker_rwsem){----..}, at: [<c0192d65>]
> shrink_slab+0x25/0x1a0 [  331.718248]  #1: 
> (&type->s_umount_key#4){-----.}, at: [<c01c21fb>]
> shrink_dcache_memory+0xfb/0x1a0 [  331.718259]
> [  331.718260] stack backtrace:
> [  331.718263] Pid: 218, comm: kswapd0 Not tainted 2.6.28-rc2-mm1-lockdep
> #6 [  331.718266] Call Trace:
> [  331.718272]  [<c0151726>] print_usage_bug+0x176/0x1c0
> [  331.718276]  [<c0152d05>] mark_lock+0xb05/0x10b0
> [  331.718282]  [<c018c0e9>] ? __free_pages_ok+0x349/0x450
> [  331.718287]  [<c0155362>] __lock_acquire+0x602/0xa80
> [  331.718291]  [<c01540ff>] ? validate_chain+0x3ef/0x1050
> [  331.718296]  [<c0155851>] lock_acquire+0x71/0xa0
> [  331.718300]  [<c01dba70>] ? inotify_inode_is_dead+0x20/0x90
> [  331.718305]  [<c037014d>] mutex_lock_nested+0x9d/0x2f0
> [  331.718310]  [<c01dba70>] ? inotify_inode_is_dead+0x20/0x90
> [  331.718314]  [<c01dba70>] ? inotify_inode_is_dead+0x20/0x90
> [  331.718318]  [<c01dba70>] inotify_inode_is_dead+0x20/0x90
> [  331.718323]  [<c024e2d6>] ? _raw_spin_unlock+0x46/0x80
> [  331.718328]  [<c01c1d14>] dentry_iput+0xa4/0xc0
> [  331.718333]  [<c01c1dfb>] d_kill+0x3b/0x60
> [  331.718337]  [<c01c1fe6>] __shrink_dcache_sb+0x1c6/0x2c0
> [  331.718342]  [<c01c228d>] shrink_dcache_memory+0x18d/0x1a0
> [  331.718347]  [<c0192e6b>] shrink_slab+0x12b/0x1a0
> [  331.718351]  [<c01939ff>] kswapd+0x3af/0x5c0
> [  331.718356]  [<c01910a0>] ? isolate_pages_global+0x0/0x220
> [  331.718362]  [<c0142800>] ? autoremove_wake_function+0x0/0x40
> [  331.718366]  [<c0193650>] ? kswapd+0x0/0x5c0
> [  331.718371]  [<c01424f7>] kthread+0x47/0x80
> [  331.718375]  [<c01424b0>] ? kthread+0x0/0x80
> [  331.718380]  [<c01054f7>] kernel_thread_helper+0x7/0x10

And here we take inode->inotify_mutex from inside __GFP_FS reclaim
context.

However, I don't think it should be possible to be the same inode
in both cases required to achieve a deadlock. Because in the
first trace, we have a reference to the dentry... I think.

Hmm, in dentry_iput, we do this

                if (!inode->i_nlink)
                        fsnotify_inoderemove(inode);

outside any locks. What is preventing races with i_nlink? Shouldn't
this logic belong in inode.c? (what, exactly, are the semantics of
this event?)

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