From 87af9e7ff9d909e70a006ca0974466e2a1d8db0a Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Fri, 12 Dec 2014 10:11:44 +0100 Subject: [PATCH] hotplugcpu: Avoid deadlocks by waking active_writer 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 removes puts_pending and turns refcount into an atomic variable. We also introduce a wait queue for the active_writer, to avoid possible races and use-after-free. There is no need to take the lock in put_online_cpus() anymore. Can't reproduce it with this fix. Signed-off-by: David Hildenbrand Signed-off-by: Paul E. McKenney --- kernel/cpu.c | 56 +++++++++++++++++++++------------------------------- 1 file changed, 23 insertions(+), 33 deletions(-) diff --git a/kernel/cpu.c b/kernel/cpu.c index 5d220234b3ca..1972b161c61e 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -58,22 +58,23 @@ static int cpu_hotplug_disabled; static struct { struct task_struct *active_writer; - struct mutex lock; /* Synchronizes accesses to refcount, */ + /* wait queue to wake up the active_writer */ + wait_queue_head_t wq; + /* verifies that no writer will get active while readers are active */ + struct mutex lock; /* * Also blocks the new readers during * an ongoing cpu hotplug operation. */ - int refcount; - /* And allows lockless put_online_cpus(). */ - atomic_t puts_pending; + atomic_t refcount; #ifdef CONFIG_DEBUG_LOCK_ALLOC struct lockdep_map dep_map; #endif } cpu_hotplug = { .active_writer = NULL, + .wq = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wq), .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), - .refcount = 0, #ifdef CONFIG_DEBUG_LOCK_ALLOC .dep_map = {.name = "cpu_hotplug.lock" }, #endif @@ -86,15 +87,6 @@ static struct { #define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map) #define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map) -static void apply_puts_pending(int max) -{ - int delta; - - if (atomic_read(&cpu_hotplug.puts_pending) >= max) { - delta = atomic_xchg(&cpu_hotplug.puts_pending, 0); - cpu_hotplug.refcount -= delta; - } -} void get_online_cpus(void) { @@ -103,8 +95,7 @@ void get_online_cpus(void) return; cpuhp_lock_acquire_read(); mutex_lock(&cpu_hotplug.lock); - apply_puts_pending(65536); - cpu_hotplug.refcount++; + atomic_inc(&cpu_hotplug.refcount); mutex_unlock(&cpu_hotplug.lock); } EXPORT_SYMBOL_GPL(get_online_cpus); @@ -116,8 +107,7 @@ bool try_get_online_cpus(void) if (!mutex_trylock(&cpu_hotplug.lock)) return false; cpuhp_lock_acquire_tryread(); - apply_puts_pending(65536); - cpu_hotplug.refcount++; + atomic_inc(&cpu_hotplug.refcount); mutex_unlock(&cpu_hotplug.lock); return true; } @@ -125,20 +115,18 @@ EXPORT_SYMBOL_GPL(try_get_online_cpus); void put_online_cpus(void) { + int refcount; + if (cpu_hotplug.active_writer == current) return; - if (!mutex_trylock(&cpu_hotplug.lock)) { - atomic_inc(&cpu_hotplug.puts_pending); - cpuhp_lock_release(); - return; - } - if (WARN_ON(!cpu_hotplug.refcount)) - cpu_hotplug.refcount++; /* try to fix things up */ + refcount = atomic_dec_return(&cpu_hotplug.refcount); + if (WARN_ON(refcount < 0)) /* try to fix things up */ + atomic_inc(&cpu_hotplug.refcount); + + if (refcount <= 0 && waitqueue_active(&cpu_hotplug.wq)) + wake_up(&cpu_hotplug.wq); - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) - wake_up_process(cpu_hotplug.active_writer); - mutex_unlock(&cpu_hotplug.lock); cpuhp_lock_release(); } @@ -168,18 +156,20 @@ EXPORT_SYMBOL_GPL(put_online_cpus); */ void cpu_hotplug_begin(void) { - cpu_hotplug.active_writer = current; + DEFINE_WAIT(wait); + cpu_hotplug.active_writer = current; cpuhp_lock_acquire(); + for (;;) { mutex_lock(&cpu_hotplug.lock); - apply_puts_pending(1); - if (likely(!cpu_hotplug.refcount)) - break; - __set_current_state(TASK_UNINTERRUPTIBLE); + prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE); + if (likely(!atomic_read(&cpu_hotplug.refcount))) + break; mutex_unlock(&cpu_hotplug.lock); schedule(); } + finish_wait(&cpu_hotplug.wq, &wait); } void cpu_hotplug_done(void) -- 2.34.1