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Date: Tue, 23 Nov 2010 14:32:08 +0100 From: Jan Kara <jack@...e.cz> To: Nick Piggin <npiggin@...nel.dk> Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, Ted Ts'o <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org Subject: Re: [patch] fix up lock order reversal in writeback On Tue 23-11-10 19:07:58, Nick Piggin wrote: > On Mon, Nov 22, 2010 at 07:16:55PM +0100, Jan Kara wrote: > > On Fri 19-11-10 16:16:19, Nick Piggin wrote: > > > On Fri, Nov 19, 2010 at 01:45:52AM +0100, Jan Kara wrote: > > > > On Wed 17-11-10 22:28:34, Andrew Morton wrote: > > > > > The fact that a call to ->write_begin can randomly return with s_umount > > > > > held, to be randomly released at some random time in the future is a > > > > > bit ugly, isn't it? write_begin is a pretty low-level, per-inode > > > > > thing. > > > > I guess you missed that writeback_inodes_sb_nr() (called from _if_idle > > > > variants) does: > > > > bdi_queue_work(sb->s_bdi, &work); > > > > wait_for_completion(&done); > > > > So we return only after all the IO has been submitted and unlock s_umount > > > > in writeback_inodes_sb_if_idle(). And we cannot really submit the IO ourselves > > > > because we are holding i_mutex and we need to get and put references > > > > to other inodes while doing writeback (those would be really horrible lock > > > > dependencies - writeback thread can put the last reference to an unlinked > > > > inode...). > > > > > > But if we're waiting for it, with the lock held, then surely it can > > > deadlock just the same as if we submit it ourself? > > Yes, that's what I realized as well and what needs fixing. It was an > > unintentional consequence of locking changes Christoph did to the writeback > > code to fix other races. > > > > > BTW. are you taking i_mutex inside writeback? I mutex can be held > > > while entering page reclaim, and thus writepage... so it could be a > > > bug too. > > No, writeback does not need i_mutex. > > I did in fact see i_mutex from writeback, which is how the lock order > reversal was noticed in the first place. I haven't had much luck in > reproducing it yet. It did come from end_io_work, I believe. > > There is another deadlock in here, by the looks (although this is not > the one which triggers lockdep -- the workqueue coupling means it > defeats lockdep detection). > > Buffered write holds i_lock, and waits for inode writeback submission, > but the work queue seems to be blocked on i_mutex (ext4_end_io_work), > and so it deadlocks. Ah, I see. That's a new thing introduced by Ted's rewrite of ext4_writepages() - bd2d0210cf22f2bd0cef72eb97cf94fc7d31d8cc. And indeed it introduced a deadlock as you describe... Honza > Note that this is an AA deadlock, different from ABBA one relating to > s_umount lock (but very similar call chains IIRC). > > > [ 748.406457] SysRq : Show Blocked State > [ 748.406685] task PC stack pid father > [ 748.406868] kworker/9:1 D 0000000000000000 6296 118 2 > 0x00000000 > [ 748.407173] ffff88012acb3c90 0000000000000046 0000000000000000 > ffff88012b1cec80 > [ 748.407686] 0000000000000002 ffff88012acb2000 ffff88012acb2000 > 000000000000d6c8 > [ 748.408200] ffff88012acb2010 ffff88012acb3fd8 ffff880129c7dd00 > ffff88012b1cec80 > [ 748.408711] Call Trace: > [ 748.408866] [<ffffffff81039431>] ? get_parent_ip+0x11/0x50 > [ 748.409033] [<ffffffff8160145c>] __mutex_lock_common+0x18c/0x480 > [ 748.409205] [<ffffffffa0042147>] ? ext4_end_io_work+0x37/0xb0 [ext4] > [ 748.409379] [<ffffffffa0042147>] ? ext4_end_io_work+0x37/0xb0 [ext4] > [ 748.409546] [<ffffffff8160182e>] mutex_lock_nested+0x3e/0x50 > [ 748.409717] [<ffffffffa0042147>] ext4_end_io_work+0x37/0xb0 [ext4] > [ 748.409883] [<ffffffff81068378>] process_one_work+0x1b8/0x5a0 > [ 748.410046] [<ffffffff8106830e>] ? process_one_work+0x14e/0x5a0 > [ 748.410219] [<ffffffffa0042110>] ? ext4_end_io_work+0x0/0xb0 [ext4] > [ 748.410386] [<ffffffff81069675>] worker_thread+0x175/0x3a0 > [ 748.410549] [<ffffffff81069500>] ? worker_thread+0x0/0x3a0 > [ 748.410712] [<ffffffff8106e246>] kthread+0x96/0xa0 > [ 748.410874] [<ffffffff81003ed4>] kernel_thread_helper+0x4/0x10 > [ 748.411038] [<ffffffff81039878>] ? finish_task_switch+0x78/0x110 > [ 748.411202] [<ffffffff816036c0>] ? restore_args+0x0/0x30 > [ 748.411364] [<ffffffff8106e1b0>] ? kthread+0x0/0xa0 > [ 748.411524] [<ffffffff81003ed0>] ? kernel_thread_helper+0x0/0x10 > [ 748.606853] dbench D 0000000000000000 2872 2916 1 > 0x00000004 > [ 748.607154] ffff880129ec58b8 0000000000000046 ffff880129ec5838 > ffffffff810806fd > [ 748.607665] ffff880129ec5898 ffff880129ec4000 ffff880129ec4000 > 000000000000d6c8 > [ 748.608176] ffff880129ec4010 ffff880129ec5fd8 ffff88010a2d0000 > ffff88011ff69f00 > [ 748.608686] Call Trace: > [ 748.608837] [<ffffffff810806fd>] ? trace_hardirqs_off+0xd/0x10 > [ 748.609001] [<ffffffff81600bb5>] schedule_timeout+0x1f5/0x350 > [ 748.609164] [<ffffffff8108380b>] ? mark_held_locks+0x6b/0xa0 > [ 748.609328] [<ffffffff8160314b>] ? _raw_spin_unlock_irq+0x2b/0x60 > [ 748.609493] [<ffffffff81039431>] ? get_parent_ip+0x11/0x50 > [ 748.609656] [<ffffffff81606c3d>] ? sub_preempt_count+0x9d/0xd0 > [ 748.609820] [<ffffffff815ffacd>] wait_for_common+0x10d/0x190 > [ 748.609984] [<ffffffff810426e0>] ? default_wake_function+0x0/0x10 > [ 748.610149] [<ffffffff81602ec9>] ? _raw_spin_unlock_bh+0x39/0x40 > [ 748.610314] [<ffffffff815ffbf8>] wait_for_completion+0x18/0x20 > [ 748.610479] [<ffffffff8113d1e7>] writeback_inodes_sb_nr+0xf7/0x120 > [ 748.610646] [<ffffffff8113d68d>] writeback_inodes_sb+0x4d/0x60 > [ 748.610811] [<ffffffff8113d6d2>] ? > writeback_inodes_sb_if_idle+0x32/0x60 > [ 748.610978] [<ffffffff8113d6da>] > writeback_inodes_sb_if_idle+0x3a/0x60 > [ 748.611153] [<ffffffffa003fefd>] ext4_da_write_begin+0x20d/0x310 > [ext4] > [ 748.611321] [<ffffffff810cbbf4>] > generic_file_buffered_write+0x114/0x2a0 > > > -- Jan Kara <jack@...e.cz> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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