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SubjectRe: [rcu] kernel BUG at include/linux/pagemap.h:149!
Hi Fengguang,

On Thu, Sep 10, 2015 at 08:57:08AM +0800, Fengguang Wu wrote:
> Greetings,
>
> 0day kernel testing robot got the below dmesg and the first bad commit is
>
> https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2015.09.01a
>
> commit d0a795e7964cca98fbefefef5e0c330b24d04f50
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> AuthorDate: Thu Jul 30 16:55:38 2015 -0700
> Commit: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CommitDate: Mon Aug 31 14:38:03 2015 -0700
>
> rcu: Don't disable preemption for Tiny and Tree RCU readers
>
> Because preempt_disable() maps to barrier() for non-debug builds,
> it forces the compiler to spill and reload registers. Because Tree
> RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
> barrier() instances generate needless extra code for each instance of
> rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree
> RCU and bloats Tiny RCU.
>
> This commit therefore removes the preempt_disable() and preempt_enable()
> from the non-preemptible implementations of __rcu_read_lock() and
> __rcu_read_unlock(), respectively.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> +------------------------------------------------+------------+------------+------------+
> | | 2d0f6efd31 | d0a795e796 | d0a795e796 |
> +------------------------------------------------+------------+------------+------------+
> | boot_successes | 63 | 0 | 0 |
> | boot_failures | 2 | 42 | 42 |
> | IP-Config:Auto-configuration_of_network_failed | 2 | | |
> | kernel_BUG_at_include/linux/pagemap.h | 0 | 42 | 42 |
> | invalid_opcode | 0 | 42 | 42 |
> | EIP_is_at_page_cache_get_speculative | 0 | 42 | 42 |
> | Kernel_panic-not_syncing:Fatal_exception | 0 | 42 | 42 |
> | backtrace:vfs_write | 0 | 42 | 42 |
> | backtrace:SyS_write | 0 | 42 | 42 |
> | backtrace:populate_rootfs | 0 | 42 | 42 |
> | backtrace:kernel_init_freeable | 0 | 42 | 42 |
> +------------------------------------------------+------------+------------+------------+
>
> dmesg for d0a795e796 and 2d0f6efd31 are both attached.
>
> [ 0.205937] PCI: CLS 0 bytes, default 32
> [ 0.206554] Unpacking initramfs...
> [ 0.208263] ------------[ cut here ]------------
> [ 0.209011] kernel BUG at include/linux/pagemap.h:149!

Code here is:

#ifdef CONFIG_TINY_RCU
# ifdef CONFIG_PREEMPT_COUNT
VM_BUG_ON(!in_atomic()); <-- BUG triggered here.
# endif
...
#endif

This indicates that CONFIG_TINY_RCU and CONFIG_PREEMPT_COUNT are both y.
Normally, IIUC, this is not possible or meaningless, because TINY_RCU is
for !PREEMPT kernel. However, according to commmit e8f7c70f4 ("sched:
Make sleeping inside spinlock detection working in !CONFIG_PREEMPT"),
maintaining preempt counts in !PREEMPT kernel makes sense for finding
preempt-related bugs.

So a possible fix would be still counting preempt_count in
rcu_read_lock and rcu_read_unlock if PREEMPT_COUNT is y for debug
purpose:

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 07f9b95..887bf5f 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -297,10 +297,16 @@ void synchronize_rcu(void);

static inline void __rcu_read_lock(void)
{
+#ifdef CONFIG_PREEMPT_COUNT
+ preempt_disable();
+#endif
}

static inline void __rcu_read_unlock(void)
{
+#ifdef CONFIG_PREEMPT_COUNT
+ preempt_enable();
+#endif
}

I did a simple booting test with the some configuration on a guest on
x86, didn't see this error again.

(Also add Frederic Weisbecker to CCed)
Regards,
Boqun

> [ 0.209033] invalid opcode: 0000 [#1] DEBUG_PAGEALLOC
> [ 0.209033] CPU: 0 PID: 1 Comm: swapper Not tainted 4.2.0-rc1-00078-gd0a795e #1
> [ 0.209033] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
> [ 0.209033] task: 8b1aa040 ti: 8b1ac000 task.ti: 8b1ac000
> [ 0.209033] EIP: 0060:[<7dccf506>] EFLAGS: 00010202 CPU: 0
> [ 0.209033] EIP is at page_cache_get_speculative+0x6c/0x149
> [ 0.209033] EAX: 00000001 EBX: 8ac00101 ECX: 00000000 EDX: 00000001
> [ 0.209033] ESI: 8bb946e0 EDI: 00000001 EBP: 8b1adc7c ESP: 8b1adc70
> [ 0.209033] DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
> [ 0.209033] CR0: 8005003b CR2: ffffffff CR3: 069fb000 CR4: 00000690
> [ 0.209033] Stack:
> [ 0.209033] 8ac0012c 8bb946e0 00000000 8b1adc9c 7dcd1219 8ad5ac7c 00000007 00000002
> [ 0.209033] 000200d2 00000000 00000000 8b1adcc0 7dcd136c 00000007 00000000 8ad5ac78
> [ 0.209033] 0000000f 000200d2 00000000 00000000 8b1adcd4 7dcd3609 000200d2 000009e8
> [ 0.209033] Call Trace:
> [ 0.209033] [<7dcd1219>] find_get_entry+0xce/0x133
> [ 0.209033] [<7dcd136c>] pagecache_get_page+0x1d/0x39d
> [ 0.209033] [<7dcd3609>] grab_cache_page_write_begin+0x3a/0x68
> [ 0.209033] [<7dd58353>] simple_write_begin+0x2d/0xbf
> [ 0.209033] [<7dcd3703>] generic_perform_write+0xcc/0x26f
> [ 0.209033] [<7dd4c3e5>] ? file_update_time+0x12b/0x135
> [ 0.209033] [<7dcd3a79>] __generic_file_write_iter+0x1d3/0x233
> [ 0.209033] [<7dcd3b2f>] generic_file_write_iter+0x56/0x128
> [ 0.209033] [<7dd2d866>] __vfs_write+0x99/0x106
> [ 0.209033] [<7dd2da75>] vfs_write+0xf7/0x136
> [ 0.209033] [<7dd2dbbc>] SyS_write+0x61/0xa7
> [ 0.209033] [<7e967bdd>] xwrite+0x23/0xa1
> [ 0.209033] [<7e967d10>] do_copy+0xb5/0xf9
> [ 0.209033] [<7e96781a>] write_buffer+0x1d/0x2c
> [ 0.209033] [<7e9679c1>] flush_buffer+0x3c/0xb0
> [ 0.209033] [<7e98e1dc>] gunzip+0x3a0/0x4b0
> [ 0.209033] [<7e98de34>] ? bunzip2+0x60c/0x60c
> [ 0.209033] [<7e98de3c>] ? nofill+0x8/0x8
> [ 0.209033] [<7e96838a>] unpack_to_rootfs+0x1b0/0x2ee
> [ 0.209033] [<7e967985>] ? error+0x2c/0x2c
> [ 0.209033] [<7e967959>] ? do_start+0x1b/0x1b
> [ 0.209033] [<7e968546>] populate_rootfs+0x7e/0x199
> [ 0.209033] [<7e966f01>] do_one_initcall+0x130/0x216
> [ 0.209033] [<7e966506>] ? repair_env_string+0x29/0x96
> [ 0.209033] [<7e9684c8>] ? unpack_to_rootfs+0x2ee/0x2ee
> [ 0.209033] [<7dc59bec>] ? parse_args+0x343/0x40a
> [ 0.209033] [<7e9670be>] ? kernel_init_freeable+0xd7/0x1b4
> [ 0.209033] [<7e9670de>] kernel_init_freeable+0xf7/0x1b4
> [ 0.209033] [<7e2736cf>] kernel_init+0x9/0x139
> [ 0.209033] [<7e281f00>] ret_from_kernel_thread+0x20/0x30
> [ 0.209033] [<7e2736c6>] ? rest_init+0x11e/0x11e
> [ 0.209033] Code: ff ff ff 7f b8 dc 98 77 7e 0f 94 c3 31 c9 0f b6 fb 89 fa e8 c7 50 fe ff 8b 04 bd dc 8a 7d 7e 40 84 db 89 04 bd dc 8a 7d 7e 74 02 <0f> 0b 8b 1e 31 c9 b8 a0 98 77 7e c1 eb 0f 83 e3 01 89 da e8 9c
> [ 0.209033] EIP: [<7dccf506>] page_cache_get_speculative+0x6c/0x149 SS:ESP 0068:8b1adc70
> [ 0.240927] ---[ end trace b15ce49b08a81922 ]---
> [ 0.241403] Kernel panic - not syncing: Fatal exception
>
> git bisect start d0a795e7964cca98fbefefef5e0c330b24d04f50 d770e558e21961ad6cfdf0ff7df0eb5d7d4f0754 --
> git bisect good 8ff4fbfd69a6c7b9598f8c1f2df34f89bac02c1a # 06:49 22+ 0 Merge branches 'fixes.2015.07.22a' and 'initexp.2015.08.04a' into HEAD
> git bisect good 8611bc8cfdb809852e15c8c8786fe0fbd72e7da7 # 06:54 22+ 0 rcu_sync: Introduce rcu_sync_dtor()
> git bisect good d74251b8bae07a4957c9f3ccecbdb7f84d790f38 # 07:03 22+ 0 locking/percpu-rwsem: Clean up the lockdep annotations in percpu_down_read()
> git bisect good 51899e3fb9b8de48dff0ab1a9cc5250c6d4020ac # 07:09 22+ 0 rcu: Use rcu_callback_t in call_rcu*() and friends
> git bisect good e8ee682bcce44c81d8363009b08f115e964dbd0b # 07:14 21+ 2 rcu: Use call_rcu_func_to to replace explicit type equivalents
> git bisect good 2d0f6efd311165fe06cecb29475e89e550b92d8c # 07:22 22+ 0 rcu: Use rsp->expedited_wq instead of sync_rcu_preempt_exp_wq
> # first bad commit: [d0a795e7964cca98fbefefef5e0c330b24d04f50] rcu: Don't disable preemption for Tiny and Tree RCU readers
> git bisect good 2d0f6efd311165fe06cecb29475e89e550b92d8c # 07:26 63+ 2 rcu: Use rsp->expedited_wq instead of sync_rcu_preempt_exp_wq
> # extra tests on HEAD of linux-devel/devel-spot-201509091835
> git bisect bad 325c0efb5f89af4e5e3d0575ea1b042375dedf6b # 07:31 0- 2 0day head guard for 'devel-spot-201509091835'
> # extra tests on tree/branch rcu/dev.2015.09.01a
> git bisect bad caa7dd68d68438b256ee830f4aa6b2ddd04b4575 # 07:36 0- 11 locktorture: Fix module unwind when bad torture_type specified
> # extra tests with first bad commit reverted
> git bisect good 9ff21ff24e5557ad2c1bd72b63f969609e0d28f8 # 07:42 66+ 2 Revert "rcu: Don't disable preemption for Tiny and Tree RCU readers"
> # extra tests on tree/branch linus/master
> git bisect good b8889c4fc6ba03e289cec6a4d692f6f080a55e53 # 07:50 66+ 0 Merge tag 'tty-4.3-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty
> # extra tests on tree/branch linux-next/master
> git bisect good 72c9d8043fdc87832802de0b7a7129d6fc4c4c70 # 07:58 63+ 0 Add linux-next specific files for 20150909
>
>


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