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Message-ID: <20120322134745.GB25897@thunk.org>
Date: Thu, 22 Mar 2012 09:47:45 -0400
From: Ted Ts'o <tytso@....edu>
To: Lukas Czerner <lczerner@...hat.com>
Cc: linux-ext4@...r.kernel.org, achender@...ux.vnet.ibm.com
Subject: Re: [PATCH 2/5] ext4: Correctly handle EOFBLOCKS flag in
ext4_ext_punch_hole
On Thu, Mar 22, 2012 at 09:25:15AM +0100, Lukas Czerner wrote:
>
> The worse what can happen is that after a write spanning several block
> we'll have first part of the write punched out, but second part written
> correctly since in this case it might hit already punched block
> and need to wait for punch_hole to finish, after that the rest of the
> range is written. However the write should remain consistent on block
> granularity which is all we guarantee anyway, right ?
I need to look more closely at this, but thing that was worrying me
was the part of truncate/punch where we have to invalidate the parts
of the page cache where we've unmapped the blocks. i.e., the call to
truncate_inode_pages_range() racing with the write. I think we're ok,
since truncate_inode_pages_range() grabs the page spinlock and then
checks for PageWriteback, which ought to be sufficient, but truncate
does take that codepath with i_mutex down, and so my spidey sense is
tingling. I may just being too paranoid, though.
Still, that's not a criticism of your patch.
More serious is the following lockdep warning that I got. Grabbing
i_mutex after the transaction handle is started can lead to a circular
locking deadlock...
- Ted
BEGIN TEST: Ext4 4k block Wed Mar 21 22:47:17 EDT 2012
Device: /dev/vdb
mke2fs options: -q
mount options: -o block_validity
000 - unknown test, ignored
FSTYP -- ext4
PLATFORM -- Linux/i686 candygram 3.3.0-rc2-00592-gc56a0b2
MKFS_OPTIONS -- -q /dev/vdc
MOUNT_OPTIONS -- -o acl,user_xattr -o block_validity /dev/vdc /vdc
075 [ 808.872903]
[ 808.873567] ======================================================
[ 808.875933] [ INFO: possible circular locking dependency detected ]
[ 808.875933] 3.3.0-rc2-00592-gc56a0b2 #32 Not tainted
[ 808.875933] -------------------------------------------------------
[ 808.875933] fsx/13769 is trying to acquire lock:
[ 808.875933] (&sb->s_type->i_mutex_key#3){+.+.+.}, at: [<c028d900>] ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933]
[ 808.875933] but task is already holding lock:
[ 808.875933] (jbd2_handle){+.+...}, at: [<c02a5995>] start_this_handle+0x4e4/0x51a
[ 808.875933]
[ 808.875933] which lock already depends on the new lock.
[ 808.875933]
[ 808.875933]
[ 808.875933] the existing dependency chain (in reverse order) is:
[ 808.875933]
[ 808.875933] -> #1 (jbd2_handle){+.+...}:
[ 808.875933] [<c019789d>] lock_acquire+0x99/0xbd
[ 808.875933] [<c02a59b7>] start_this_handle+0x506/0x51a
[ 808.875933] [<c02a5ba6>] jbd2__journal_start+0xae/0xda
[ 808.875933] [<c02a5be4>] jbd2_journal_start+0x12/0x14
[ 808.875933] [<c0284fb8>] ext4_journal_start_sb+0x11e/0x126
[ 808.875933] [<c0277661>] ext4_unlink+0x82/0x1e5
[ 808.875933] [<c02127e1>] vfs_unlink+0x61/0xaf
[ 808.875933] [<c02147b5>] do_unlinkat+0xa0/0x112
[ 808.875933] [<c0214946>] sys_unlinkat+0x30/0x37
[ 808.875933] [<c06d8c5d>] syscall_call+0x7/0xb
[ 808.875933]
[ 808.875933] -> #0 (&sb->s_type->i_mutex_key#3){+.+.+.}:
[ 808.875933] [<c0197598>] __lock_acquire+0x989/0xbf5
[ 808.875933] [<c019789d>] lock_acquire+0x99/0xbd
[ 808.875933] [<c06d65f4>] __mutex_lock_common+0x30/0x316
[ 808.875933] [<c06d6988>] mutex_lock_nested+0x26/0x2f
[ 808.875933] [<c028d900>] ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933] [<c026e316>] ext4_punch_hole+0x5f/0x70
[ 808.875933] [<c028fbce>] ext4_fallocate+0x63/0x469
[ 808.875933] [<c0208974>] do_fallocate+0xe7/0x105
[ 808.875933] [<c02089c3>] sys_fallocate+0x31/0x46
[ 808.875933] [<c06d8c5d>] syscall_call+0x7/0xb
[ 808.875933]
[ 808.875933] other info that might help us debug this:
[ 808.875933]
[ 808.875933] Possible unsafe locking scenario:
[ 808.875933]
[ 808.875933] CPU0 CPU1
[ 808.875933] ---- ----
[ 808.875933] lock(jbd2_handle);
[ 808.875933] lock(&sb->s_type->i_mutex_key#3);
[ 808.875933] lock(jbd2_handle);
[ 808.875933] lock(&sb->s_type->i_mutex_key#3);
[ 808.875933]
[ 808.875933] *** DEADLOCK ***
[ 808.875933]
[ 808.875933] 1 lock held by fsx/13769:
[ 808.875933] #0: (jbd2_handle){+.+...}, at: [<c02a5995>] start_this_handle+0x4e4/0x51a
[ 808.875933]
[ 808.875933] stack backtrace:
[ 808.875933] Pid: 13769, comm: fsx Not tainted 3.3.0-rc2-00592-gc56a0b2 #32
[ 808.875933] Call Trace:
[ 808.875933] [<c01954fb>] print_circular_bug+0x194/0x1a1
[ 808.875933] [<c0197598>] __lock_acquire+0x989/0xbf5
[ 808.875933] [<c019789d>] lock_acquire+0x99/0xbd
[ 808.875933] [<c028d900>] ? ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933] [<c06d65f4>] __mutex_lock_common+0x30/0x316
[ 808.875933] [<c028d900>] ? ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933] [<c017d53a>] ? local_clock+0x3d/0x55
[ 808.875933] [<c01942de>] ? lock_release_holdtime+0x2b/0xcd
[ 808.875933] [<c028d8d9>] ? ext4_ext_punch_hole+0x291/0x382
[ 808.875933] [<c06d6988>] mutex_lock_nested+0x26/0x2f
[ 808.875933] [<c028d900>] ? ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933] [<c028d900>] ext4_ext_punch_hole+0x2b8/0x382
[ 808.875933] [<c026e316>] ext4_punch_hole+0x5f/0x70
[ 808.875933] [<c028fbce>] ext4_fallocate+0x63/0x469
[ 808.875933] [<c017d4ed>] ? sched_clock_cpu+0x134/0x144
[ 808.875933] [<c023473e>] ? fsnotify+0x1e8/0x202
[ 808.875933] [<c01940d5>] ? trace_hardirqs_off+0xb/0xd
[ 808.875933] [<c017d53a>] ? local_clock+0x3d/0x55
[ 808.875933] [<c020a873>] ? fget+0x57/0x71
[ 808.875933] [<c0208974>] do_fallocate+0xe7/0x105
[ 808.875933] [<c02089c3>] sys_fallocate+0x31/0x46
[ 808.875933] [<c06d8c5d>] syscall_call+0x7/0xb
[ 808.875933] [<c06d0000>] ? init_intel+0x1aa/0x370
--
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