Messages in this thread |  | | Date | Wed, 19 Mar 2014 11:08:08 +0800 | From | Jane Li <> | Subject | Re: [PATCH] printk: fix one circular lockdep warning about console_lock |
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On 02/12/2014 05:19 AM, Andrew Morton wrote:
> On Tue, 11 Feb 2014 14:50:00 +0800<jiel@marvell.com> wrote: > >> From: Jane Li<jiel@marvell.com> >> >> This patch tries to fix a warning about possible circular locking >> dependency. >> >> If do in following sequence: >> enter suspend -> resume -> plug-out CPUx (echo 0 > cpux/online) >> lockdep will show warning as following: >> >> ====================================================== >> [ INFO: possible circular locking dependency detected ] >> 3.10.0 #2 Tainted: G O >> ------------------------------------------------------- >> sh/1271 is trying to acquire lock: >> (console_lock){+.+.+.}, at: [<c06ebf7c>] console_cpu_notify+0x20/0x2c >> but task is already holding lock: >> (cpu_hotplug.lock){+.+.+.}, at: [<c012b4e8>] cpu_hotplug_begin+0x2c/0x58 >> which lock already depends on the new lock. >> >> the existing dependency chain (in reverse order) is: >> -> #2 (cpu_hotplug.lock){+.+.+.}: >> [<c017bb7c>] lock_acquire+0x98/0x12c >> [<c06f5014>] mutex_lock_nested+0x50/0x3d8 >> [<c012b4e8>] cpu_hotplug_begin+0x2c/0x58 >> [<c06ebfac>] _cpu_up+0x24/0x154 >> [<c06ec140>] cpu_up+0x64/0x84 >> [<c0981834>] smp_init+0x9c/0xd4 >> [<c0973880>] kernel_init_freeable+0x78/0x1c8 >> [<c06e7f40>] kernel_init+0x8/0xe4 >> [<c010eec8>] ret_from_fork+0x14/0x2c >> >> -> #1 (cpu_add_remove_lock){+.+.+.}: >> [<c017bb7c>] lock_acquire+0x98/0x12c >> [<c06f5014>] mutex_lock_nested+0x50/0x3d8 >> [<c012b758>] disable_nonboot_cpus+0x8/0xe8 >> [<c016b83c>] suspend_devices_and_enter+0x214/0x448 >> [<c016bc54>] pm_suspend+0x1e4/0x284 >> [<c016bdcc>] try_to_suspend+0xa4/0xbc >> [<c0143848>] process_one_work+0x1c4/0x4fc >> [<c0143f80>] worker_thread+0x138/0x37c >> [<c014aaf8>] kthread+0xa4/0xb0 >> [<c010eec8>] ret_from_fork+0x14/0x2c >> >> -> #0 (console_lock){+.+.+.}: >> [<c017b5d0>] __lock_acquire+0x1b38/0x1b80 >> [<c017bb7c>] lock_acquire+0x98/0x12c >> [<c01288c4>] console_lock+0x54/0x68 >> [<c06ebf7c>] console_cpu_notify+0x20/0x2c >> [<c01501d4>] notifier_call_chain+0x44/0x84 >> [<c012b448>] __cpu_notify+0x2c/0x48 >> [<c012b5b0>] cpu_notify_nofail+0x8/0x14 >> [<c06e81bc>] _cpu_down+0xf4/0x258 >> [<c06e8344>] cpu_down+0x24/0x40 >> [<c06e921c>] store_online+0x30/0x74 >> [<c03b7298>] dev_attr_store+0x18/0x24 >> [<c025fc5c>] sysfs_write_file+0x16c/0x19c >> [<c0207a98>] vfs_write+0xb4/0x190 >> [<c0207e58>] SyS_write+0x3c/0x70 >> [<c010ee00>] ret_fast_syscall+0x0/0x48 >> >> Chain exists of: >> console_lock --> cpu_add_remove_lock --> cpu_hotplug.lock >> >> Possible unsafe locking scenario: >> CPU0 CPU1 >> ---- ---- >> lock(cpu_hotplug.lock); >> lock(cpu_add_remove_lock); >> lock(cpu_hotplug.lock); >> lock(console_lock); >> *** DEADLOCK *** > These traces hurt my brain. > >> There are three locks involved in two sequence: >> a) pm suspend: >> console_lock (@suspend_console()) >> cpu_add_remove_lock (@disable_nonboot_cpus()) >> cpu_hotplug.lock (@_cpu_down()) > But but but. suspend_console() releases console_sem again. So the > sequence is actually > > down(&console_sem) (@suspend_console()) > up(&console_sem) (@suspend_console()) > cpu_add_remove_lock (@disable_nonboot_cpus()) > cpu_hotplug.lock (@_cpu_down()) > > So console_sem *doesn't* nest outside cpu_add_remove_lock and > cpu_hotplug.lock.
Jan Kara and Jane have answered this question in other emails.
>> b) Plug-out CPUx: >> cpu_add_remove_lock (@(cpu_down()) >> cpu_hotplug.lock (@_cpu_down()) >> console_lock (@console_cpu_notify()) => Lockdeps prints warning log. >> >> There should be not real deadlock, as flag of console_suspended can >> protect this. > console_lock() does down(&console_sem) *before* testing > console_suspended, so I don't understand this sentence - a more > detailed description would help.
Jane has answered this question in another email.
>> Printk registers cpu hotplug notify function. When CPUx is plug-out/in, >> always execute console_lock() and console_unlock(). This patch >> modifies that with console_trylock() and console_unlock(). Then use >> that instead of the unconditional console_lock/unlock pair to avoid the >> warning. >> >> ... >> >> --- a/kernel/printk/printk.c >> +++ b/kernel/printk/printk.c >> @@ -1893,6 +1893,20 @@ void resume_console(void) >> } >> >> /** >> + * console_flush - flush dmesg if console isn't suspended >> + * >> + * console_unlock always flushes the dmesg buffer, so just try to >> + * grab&drop the console lock. If that fails we know that the current >> + * holder will eventually drop the console lock and so flush the dmesg >> + * buffers at the earliest possible time. >> + */ > The comment should describe why we added this code, please: talk about > cpu_hotplug.lock and console_lock.
Daniel has answered this question in another email.
>> +void console_flush(void) >> +{ >> + if (console_trylock()) >> + console_unlock(); >> +} >> + >> +/** >> * console_cpu_notify - print deferred console messages after CPU hotplug >> * @self: notifier struct >> * @action: CPU hotplug event >> @@ -1911,8 +1925,7 @@ static int console_cpu_notify(struct notifier_block *self, >> case CPU_DEAD: >> case CPU_DOWN_FAILED: >> case CPU_UP_CANCELED: >> - console_lock(); >> - console_unlock(); >> + console_flush(); >> } >> return NOTIFY_OK; > Well, this is a bit hacky and makes the already-far-too-complex code > even more complex. If it is indeed the case that the deadlock cannot > really occur then let's try to find a way of suppressing the lockdep > warning without making runtime changes. > > What I'm struggling with is what *should* the ranking of these locks be? > From a conceptual high-level design standpoint, which is the > "innermost" lock? I tend to think that it is console_lock, because > blocking CPU hotplug is a quite high-level operation. > > But console_lock is such a kooky special-case in the way it is used to > control the printk corking that it is hard to take general rules and > apply them here.
Daniel and Jan Kara have answered this question in other emails.
Do you agree with this solution or have other comments?
Thanks!
Best Regards, Jane
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