Messages in this thread Patch in this message |  | | Date | Wed, 19 Mar 2014 11:00:05 +0100 | From | Jan Kara <> | Subject | Re: [PATCH] printk: fix one circular lockdep warning about console_lock |
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On Wed 19-03-14 11:08:08, Jane Li wrote: > 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? Umm, I disagree with the patch. What I proposed in my answer to your patch is something like the patch below. Does it fix your problem?
Honza
From 91497a88c403a7f22e78fee2f69d7413c6e8209f Mon Sep 17 00:00:00 2001 From: Jan Kara <jack@suse.cz> Date: Wed, 19 Mar 2014 10:56:12 +0100 Subject: [PATCH] printk: Fixup lockdep annotation in console_suspend()
Although console_suspend() releases console_sem, it doesn't tell lockdep about it. That results in the lockdep warning about circular locking when doing the following: enter suspend -> resume -> plug-out CPUx (echo 0 > cpux/online)
Fix the problem by telling lockdep we actually released the semaphore in console_suspend() and acquired it again in console_resume().
Signed-off-by: Jan Kara <jack@suse.cz> --- kernel/printk/printk.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 4dae9cbe9259..e6ada322782b 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1880,6 +1880,7 @@ void suspend_console(void) console_lock(); console_suspended = 1; up(&console_sem); + mutex_release(&console_lock_dep_map, 1, _RET_IP_); } void resume_console(void) @@ -1887,6 +1888,7 @@ void resume_console(void) if (!console_suspend_enabled) return; down(&console_sem); + mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_); console_suspended = 0; console_unlock(); } -- 1.8.1.4
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