[PATCH] timer/hrtimer: take per cpu locks in sane order (e81ce1f7) · Commits · 戴 / test

Doing something like this on a two cpu system # echo 0 > /sys/devices/system/cpu/cpu0/online # echo 1 > /sys/devices/system/cpu/cpu0/online # echo 0 > /sys/devices/system/cpu/cpu1/online will give me this: ======================================================= [ INFO: possible circular locking dependency detected ] 2.6.21-rc2-g562aa1d4-dirty #7 ------------------------------------------------------- bash/1282 is trying to acquire lock: (&cpu_base->lock_key){.+..}, at: [<000000000005f17e>] hrtimer_cpu_notify+0xc6/0x240 but task is already holding lock: (&cpu_base->lock_key#2){.+..}, at: [<000000000005f174>] hrtimer_cpu_notify+0xbc/0x240 which lock already depends on the new lock. This happens because we have the following code in kernel/hrtimer.c: migrate_hrtimers(int cpu) [...] old_base = &per_cpu(hrtimer_bases, cpu); new_base = &get_cpu_var(hrtimer_bases); [...] spin_lock(&new_base->lock); spin_lock(&old_base->lock); Which means the spinlocks are taken in an order which depends on which cpu gets shut down from which other cpu. Therefore lockdep complains that there might be an ABBA deadlock. Since migrate_hrtimers() gets only called on cpu hotplug it's safe to assume that it isn't executed concurrently on a The same problem exists in kernel/timer.c: migrate_timers(). As pointed out by Christian Borntraeger one possible solution to avoid the locking order complaints would be to make sure that the locks are always taken in the same order. E.g. by taking the lock of the cpu with the lower number first. To achieve this we introduce two new spinlock functions double_spin_lock and double_spin_unlock which lock or unlock two locks in a given order. Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: John Stultz <johnstul@us.ibm.com> Cc: Christian Borntraeger <cborntra@de.ibm.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Signed-off-by:

Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

include/linux/spinlock.h

+37 −0

Original line number	Diff line number	Diff line
		@@ -282,6 +282,43 @@ do { \
		1 : ({ local_irq_restore(flags); 0; }); \
		})

		/*
		* Locks two spinlocks l1 and l2.
		* l1_first indicates if spinlock l1 should be taken first.
		*/
		static inline void double_spin_lock(spinlock_t l1, spinlock_t l2,
		bool l1_first)
		__acquires(l1)
		__acquires(l2)
		{
		if (l1_first) {
		spin_lock(l1);
		spin_lock(l2);
		} else {
		spin_lock(l2);
		spin_lock(l1);
		}
		}

		/*
		* Unlocks two spinlocks l1 and l2.
		* l1_taken_first indicates if spinlock l1 was taken first and therefore
		* should be released after spinlock l2.
		*/
		static inline void double_spin_unlock(spinlock_t l1, spinlock_t l2,
		bool l1_taken_first)
		__releases(l1)
		__releases(l2)
		{
		if (l1_taken_first) {
		spin_unlock(l2);
		spin_unlock(l1);
		} else {
		spin_unlock(l1);
		spin_unlock(l2);
		}
		}

		/*
		* Pull the atomic_t declaration:
		* (asm-mips/atomic.h needs above definitions)

kernel/hrtimer.c

+4 −5

Original line number	Diff line number	Diff line
		@@ -1355,17 +1355,16 @@ static void migrate_hrtimers(int cpu)
		tick_cancel_sched_timer(cpu);

		local_irq_disable();

		spin_lock(&new_base->lock);
		spin_lock(&old_base->lock);
		double_spin_lock(&new_base->lock, &old_base->lock,
		smp_processor_id() < cpu);

		for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
		migrate_hrtimer_list(&old_base->clock_base[i],
		&new_base->clock_base[i]);
		}
		spin_unlock(&old_base->lock);
		spin_unlock(&new_base->lock);

		double_spin_unlock(&new_base->lock, &old_base->lock,
		smp_processor_id() < cpu);
		local_irq_enable();
		put_cpu_var(hrtimer_bases);
		}

kernel/timer.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -1651,8 +1651,8 @@ static void __devinit migrate_timers(int cpu)
		new_base = get_cpu_var(tvec_bases);

		local_irq_disable();
		spin_lock(&new_base->lock);
		spin_lock(&old_base->lock);
		double_spin_lock(&new_base->lock, &old_base->lock,
		smp_processor_id() < cpu);

		BUG_ON(old_base->running_timer);

		@@ -1665,8 +1665,8 @@ static void __devinit migrate_timers(int cpu)
		migrate_timer_list(new_base, old_base->tv5.vec + i);
		}

		spin_unlock(&old_base->lock);
		spin_unlock(&new_base->lock);
		double_spin_unlock(&new_base->lock, &old_base->lock,
		smp_processor_id() < cpu);
		local_irq_enable();
		put_cpu_var(tvec_bases);
		}

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