From 90d45d17f3e68608ac7ba8fc3d7acce022a19c8e Mon Sep 17 00:00:00 2001 From: Ashok Raj Date: Tue, 8 Nov 2005 21:34:24 -0800 Subject: [PATCH] cpu hotplug: fix locking in cpufreq drivers When calling target drivers to set frequency, we take cpucontrol lock. When we modified the code to accomodate CPU hotplug, there was an attempt to take a double lock of cpucontrol leading to a deadlock. Since the current thread context is already holding the cpucontrol lock, we dont need to make another attempt to acquire it. Now we leave a trace in current->flags indicating current thread already is under cpucontrol lock held, so we dont attempt to do this another time. Thanks to Andrew Morton for the beating:-) From: Brice Goglin Build fix (akpm: this patch is still unpleasant. Ashok continues to look for a cleaner solution, doesn't he? ;)) Signed-off-by: Ashok Raj Signed-off-by: Brice Goglin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/cpu.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) (limited to 'kernel') diff --git a/kernel/cpu.c b/kernel/cpu.c index 3619e939182..d61ba88f34e 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -21,6 +21,24 @@ EXPORT_SYMBOL_GPL(cpucontrol); static struct notifier_block *cpu_chain; +/* + * Used to check by callers if they need to acquire the cpucontrol + * or not to protect a cpu from being removed. Its sometimes required to + * call these functions both for normal operations, and in response to + * a cpu being added/removed. If the context of the call is in the same + * thread context as a CPU hotplug thread, we dont need to take the lock + * since its already protected + * check drivers/cpufreq/cpufreq.c for its usage - Ashok Raj + */ + +int current_in_cpu_hotplug(void) +{ + return (current->flags & PF_HOTPLUG_CPU); +} + +EXPORT_SYMBOL_GPL(current_in_cpu_hotplug); + + /* Need to know about CPUs going up/down? */ int register_cpu_notifier(struct notifier_block *nb) { @@ -94,6 +112,13 @@ int cpu_down(unsigned int cpu) goto out; } + /* + * Leave a trace in current->flags indicating we are already in + * process of performing CPU hotplug. Callers can check if cpucontrol + * is already acquired by current thread, and if so not cause + * a dead lock by not acquiring the lock + */ + current->flags |= PF_HOTPLUG_CPU; err = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE, (void *)(long)cpu); if (err == NOTIFY_BAD) { @@ -146,6 +171,7 @@ out_thread: out_allowed: set_cpus_allowed(current, old_allowed); out: + current->flags &= ~PF_HOTPLUG_CPU; unlock_cpu_hotplug(); return err; } @@ -163,6 +189,12 @@ int __devinit cpu_up(unsigned int cpu) ret = -EINVAL; goto out; } + + /* + * Leave a trace in current->flags indicating we are already in + * process of performing CPU hotplug. + */ + current->flags |= PF_HOTPLUG_CPU; ret = notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu); if (ret == NOTIFY_BAD) { printk("%s: attempt to bring up CPU %u failed\n", @@ -185,6 +217,7 @@ out_notify: if (ret != 0) notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu); out: + current->flags &= ~PF_HOTPLUG_CPU; up(&cpucontrol); return ret; } -- cgit v1.2.3