Greetings,

I got this warning when doing writeback tests. It happens quite often
in 3.6.0-rc1 as well as in 3.5.0.

[  644.273874] btrfs: block rsv returned -28
[  644.278161] ------------[ cut here ]------------
[  644.283050] WARNING: at /c/wfg/tip/fs/btrfs/extent-tree.c:6330 btrfs_alloc_free_block+0xd6/0x28b()
[  644.292468] Hardware name: X7DW3
[  644.295950] Modules linked in:
[  644.299330] Pid: 10214, comm: btrfs-transacti Not tainted 3.6.0-rc1+ #4
[  644.306201] Call Trace:
[  644.308916]  [<ffffffff810697ee>] warn_slowpath_common+0x83/0x9c
[  644.315183]  [<ffffffff81069821>] warn_slowpath_null+0x1a/0x1c
[  644.321274]  [<ffffffff8133246f>] btrfs_alloc_free_block+0xd6/0x28b
[  644.327796]  [<ffffffff81141973>] ? kfree+0x55/0x101
[  644.333020]  [<ffffffff813218ac>] __btrfs_cow_block+0x118/0x43d
[  644.339196]  [<ffffffff8133c00e>] ? btrfs_buffer_uptodate+0x49/0x5f
[  644.345717]  [<ffffffff81321d17>] btrfs_cow_block+0x146/0x1ac
[  644.351722]  [<ffffffff81367396>] ? btrfs_set_lock_blocking_rw+0xa9/0xf7
[  644.358677]  [<ffffffff813253a6>] btrfs_search_slot+0x2e1/0x784
[  644.364853]  [<ffffffff8133753f>] btrfs_del_csums+0xee/0x2e2
[  644.370772]  [<ffffffff8132cedd>] __btrfs_free_extent+0x5b3/0x60b
[  644.377129]  [<ffffffff81330c5d>] run_clustered_refs+0x79b/0x85f
[  644.383390]  [<ffffffff81330fb3>] btrfs_run_delayed_refs+0x292/0x3ed
[  644.390001]  [<ffffffff8197c2eb>] ? mutex_unlock+0xe/0x10
[  644.395658]  [<ffffffff81352284>] ? btrfs_run_ordered_operations+0x1a4/0x1b3
[  644.402966]  [<ffffffff813400b6>] btrfs_commit_transaction+0x94/0x995
[  644.409664]  [<ffffffff8108a996>] ? __init_waitqueue_head+0x33/0x33
[  644.416185]  [<ffffffff8107757a>] ? usleep_range+0x42/0x42
[  644.421931]  [<ffffffff8133a0bc>] transaction_kthread+0xec/0x1b3
[  644.428192]  [<ffffffff81339fd0>] ? cleaner_kthread+0xe8/0xe8
[  644.434195]  [<ffffffff8108a09b>] kthread+0x8b/0x93
[  644.439333]  [<ffffffff819863c4>] kernel_thread_helper+0x4/0x10
[  644.445509]  [<ffffffff8197e7dd>] ? retint_restore_args+0x13/0x13
[  644.451857]  [<ffffffff8108a010>] ? __init_kthread_worker+0x39/0x39
[  644.458379]  [<ffffffff819863c0>] ? gs_change+0x13/0x13
[  644.463861] ---[ end trace 10c6b128620d1b3b ]---
[  644.468767] btrfs: block rsv returned -28
[  644.473041] ------------[ cut here ]------------
[  644.477917] WARNING: at /c/wfg/tip/fs/btrfs/extent-tree.c:6330 btrfs_alloc_free_block+0xd6/0x28b()
[  644.487339] Hardware name: X7DW3
[  644.490828] Modules linked in:
[  644.494214] Pid: 10214, comm: btrfs-transacti Tainted: G        W    3.6.0-rc1+ #4
[  644.502254] Call Trace:
[  644.504957]  [<ffffffff810697ee>] warn_slowpath_common+0x83/0x9c
[  644.511221]  [<ffffffff81069821>] warn_slowpath_null+0x1a/0x1c
[  644.517314]  [<ffffffff8133246f>] btrfs_alloc_free_block+0xd6/0x28b
[  644.523833]  [<ffffffff81141973>] ? kfree+0x55/0x101
[  644.529057]  [<ffffffff813218ac>] __btrfs_cow_block+0x118/0x43d
[  644.535235]  [<ffffffff8133c00e>] ? btrfs_buffer_uptodate+0x49/0x5f
[  644.541756]  [<ffffffff81321d17>] btrfs_cow_block+0x146/0x1ac
[  644.547761]  [<ffffffff81367396>] ? btrfs_set_lock_blocking_rw+0xa9/0xf7
[  644.554714]  [<ffffffff813253a6>] btrfs_search_slot+0x2e1/0x784
[  644.560892]  [<ffffffff8133753f>] btrfs_del_csums+0xee/0x2e2
[  644.566811]  [<ffffffff8132cedd>] __btrfs_free_extent+0x5b3/0x60b
[  644.573168]  [<ffffffff81330c5d>] run_clustered_refs+0x79b/0x85f
[  644.579439]  [<ffffffff81330fb3>] btrfs_run_delayed_refs+0x292/0x3ed
[  644.586057]  [<ffffffff8197c2eb>] ? mutex_unlock+0xe/0x10
[  644.591712]  [<ffffffff81352284>] ? btrfs_run_ordered_operations+0x1a4/0x1b3
[  644.599016]  [<ffffffff813400b6>] btrfs_commit_transaction+0x94/0x995
[  644.605709]  [<ffffffff8108a996>] ? __init_waitqueue_head+0x33/0x33
[  644.612233]  [<ffffffff8107757a>] ? usleep_range+0x42/0x42
[  644.617979]  [<ffffffff8133a0bc>] transaction_kthread+0xec/0x1b3
[  644.624239]  [<ffffffff81339fd0>] ? cleaner_kthread+0xe8/0xe8
[  644.630241]  [<ffffffff8108a09b>] kthread+0x8b/0x93
[  644.635378]  [<ffffffff819863c4>] kernel_thread_helper+0x4/0x10
[  644.641555]  [<ffffffff8197e7dd>] ? retint_restore_args+0x13/0x13
[  644.647905]  [<ffffffff8108a010>] ? __init_kthread_worker+0x39/0x39
[  644.654428]  [<ffffffff819863c0>] ? gs_change+0x13/0x13
[  644.659910] ---[ end trace 10c6b128620d1b3c ]---
[  651.834616] use_block_rsv: 2039 callbacks suppressed

Thanks,
Fengguang