On Tue, Dec 09, 2014 at 01:23:31PM +0100, David Hildenbrand wrote:
> Commit b2c4623dcd07 ("rcu: More on deadlock between CPU hotplug and expedited
> grace periods") introduced another problem that can easily be reproduced by
> starting/stopping cpus in a loop.
>
> E.g.:
> for i in `seq 5000`; do
> echo 1 > /sys/devices/system/cpu/cpu1/online
> echo 0 > /sys/devices/system/cpu/cpu1/online
> done
>
> Will result in:
> INFO: task /cpu_start_stop:1 blocked for more than 120 seconds.
> Call Trace:
> ([<00000000006a028e>] __schedule+0x406/0x91c)
> [<0000000000130f60>] cpu_hotplug_begin+0xd0/0xd4
> [<0000000000130ff6>] _cpu_up+0x3e/0x1c4
> [<0000000000131232>] cpu_up+0xb6/0xd4
> [<00000000004a5720>] device_online+0x80/0xc0
> [<00000000004a57f0>] online_store+0x90/0xb0
> ...
>
> And a deadlock.
>
> Problem is that if the last ref in put_online_cpus() can't get the
> cpu_hotplug.lock the puts_pending count is incremented, but a sleeping
> active_writer might never be woken up, therefore never exiting the loop in
> cpu_hotplug_begin().
>
> This fix wakes up the active_writer proactively. The writer already goes back
> to
> sleep if the ref count isn't already down to 0, so this should be fine.
>
> In order to avoid many potential races, we have to:
> - Protect current_writer by a spin lock. When holding this lock we can be sure
> that the writer won't vainsh or change. (use-after-free)
> - Increment the cpu_hotplug.puts_pending count before we test for an
> active_writer. (otherwise a wakeup might get lost)
> - Move setting of TASK_UNINTERRUPTIBLE in cpu_hotplug_begin() above the
> condition check. (otherwise a wakeup might get lost)
>
> Can't reproduce it with this fix.
Would wait_event()/wake_up() work for the wakeup-writer case?
Thanx, Paul
> Signed-off-by: David Hildenbrand <d...@linux.vnet.ibm.com>
> ---
> kernel/cpu.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 90a3d01..7489b7a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -58,6 +58,7 @@ static int cpu_hotplug_disabled;
>
> static struct {
> struct task_struct *active_writer;
> + spinlock_t awr_lock; /* protects active_writer from being changed */
> struct mutex lock; /* Synchronizes accesses to refcount, */
> /*
> * Also blocks the new readers during
> @@ -72,6 +73,7 @@ static struct {
> #endif
> } cpu_hotplug = {
> .active_writer = NULL,
> + .awr_lock = __SPIN_LOCK_UNLOCKED(cpu_hotplug.awr_lock),
> .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
> .refcount = 0,
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> @@ -116,7 +118,13 @@ void put_online_cpus(void)
> if (cpu_hotplug.active_writer == current)
> return;
> if (!mutex_trylock(&cpu_hotplug.lock)) {
> + /* inc before testing for active_writer to not lose wake ups */
> atomic_inc(&cpu_hotplug.puts_pending);
> + spin_lock(&cpu_hotplug.awr_lock);
> + /* we might be the last one */
> + if (unlikely(cpu_hotplug.active_writer))
> + wake_up_process(cpu_hotplug.active_writer);
> + spin_unlock(&cpu_hotplug.awr_lock);
> cpuhp_lock_release();
> return;
> }
> @@ -156,20 +164,24 @@ EXPORT_SYMBOL_GPL(put_online_cpus);
> */
> void cpu_hotplug_begin(void)
> {
> + spin_lock(&cpu_hotplug.awr_lock);
> cpu_hotplug.active_writer = current;
> + spin_unlock(&cpu_hotplug.awr_lock);
>
> cpuhp_lock_acquire();
> for (;;) {
> mutex_lock(&cpu_hotplug.lock);
> + __set_current_state(TASK_UNINTERRUPTIBLE);
> if (atomic_read(&cpu_hotplug.puts_pending)) {
> int delta;
>
> delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
> cpu_hotplug.refcount -= delta;
> }
> - if (likely(!cpu_hotplug.refcount))
> + if (likely(!cpu_hotplug.refcount)) {
> + __set_current_state(TASK_RUNNING);
> break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> + }
> mutex_unlock(&cpu_hotplug.lock);
> schedule();
> }
> @@ -177,7 +189,9 @@ void cpu_hotplug_begin(void)
>
> void cpu_hotplug_done(void)
> {
> + spin_lock(&cpu_hotplug.awr_lock);
> cpu_hotplug.active_writer = NULL;
> + spin_unlock(&cpu_hotplug.awr_lock);
> mutex_unlock(&cpu_hotplug.lock);
> cpuhp_lock_release();
> }
> --
> 1.8.5.5
>
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