Messages in this thread Patch in this message |  | | From | <> | Subject | [PATCH] printk: fix one circular lockdep warning about console_lock | Date | Tue, 11 Feb 2014 14:50:00 +0800 |
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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 ***
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()) 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.
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.
Signed-off-by: Jane Li <jiel@marvell.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> --- kernel/printk/printk.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index b1d255f..e7a0525 100644 --- 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. + */ +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; } -- 1.7.9.5
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