Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/20/2011 03:07 PM, mengcong wrote:

> On Tue, 2011-12-20 at 12:58 +0530, Srivatsa S. Bhat wrote:
>> On 12/20/2011 11:57 AM, Al Viro wrote:
>>
>>> On Tue, Dec 20, 2011 at 10:26:05AM +0530, Srivatsa S. Bhat wrote:
>>>> Oh, right, that has to be handled as well...
>>>>
>>>> Hmmm... How about registering a CPU hotplug notifier callback during lock init
>>>> time, and then for every cpu that gets onlined (after we took a copy of the
>>>> cpu_online_mask to work with), we see if that cpu is different from the ones
>>>> we have already locked, and if it is, we lock it in the callback handler and
>>>> update the locked_cpu_mask appropriately (so that we release the locks properly
>>>> during the unlock operation).
>>>>
>>>> Handling the newly introduced race between the callback handler and lock-unlock
>>>> code must not be difficult, I believe..
>>>>
>>>> Any loopholes in this approach? Or is the additional complexity just not worth
>>>> it here?
>>>
>>> To summarize the modified variant of that approach hashed out on IRC:
>>>
>>> 	* lglock grows three extra things: spinlock, cpu bitmap and cpu hotplug
>>> notifier.
>>> 	* foo_global_lock_online starts with grabbing that spinlock and
>>> loops over the cpus in that bitmap.
>>> 	* foo_global_unlock_online loops over the same bitmap and then drops
>>> that spinlock
>>> 	* callback of the notifier is going to do all bitmap updates.  Under
>>> that spinlock.  Events that need handling definitely include the things like
>>> "was going up but failed", since we need the bitmap to contain all online CPUs
>>> at all time, preferably without too much junk beyond that.  IOW, we need to add
>>> it there _before_ low-level __cpu_up() calls set_cpu_online().  Which means
>>> that we want to clean up on failed attempt to up it.  Taking a CPU down is
>>> probably less PITA; just clear bit on the final "the sucker's dead" event.
>>> 	* bitmap is initialized once, at the same time we set the notifier
>>> up.  Just grab the spinlock and do
>>> 	for_each_online_cpu(N)
>>> 		add N to bitmap
>>> then release the spinlock and let the callbacks handle all updates.
>>>
>>> I think that'll work with relatively little pain, but I'm not familiar enough
>>> with the cpuhotplug notifiers, so I'd rather have the folks familiar with those
>>> to supply the set of events to watch for...
>>>
>>
>>
>> We need not watch out for "up failed" events. It is enough if we handle
>> CPU_ONLINE and CPU_DEAD events. Because, these 2 events are triggered only
>> upon successful online or offline operation, and these notifications are
>> more than enough for our purpose (to update our bitmaps). Also, those cpus
>> which came online wont start running until these "success notifications"
>> are all done, which is where we do our stuff in the callback (ie., try
>> grabbing the spinlock..).
>>
>> Of course, by doing this (only looking out for CPU_ONLINE and CPU_DEAD
>> events), our bitmap will probably be one step behind cpu_online_mask
>> (which means, we'll still have to take the snapshot of cpu_online_mask and
>> work with it instead of using for_each_online_cpu()).
>> But that doesn't matter, as long as:
>>   * we don't allow the newly onlined CPU to start executing code (this
>>     is achieved by taking the spinlock in the callback)
> 
> I think cpu notifier callback doesn't always run on the UPing cpu.
> Actually, it rarely runs on the UPing cpu.
> If I was wrong about the above thought, there is still a chance that lg-lock
> operations are scheduled on the UPing cpu before calling the callback.
> 


I wasn't actually banking on that, but you have raised a very good point.
The scheduler uses its own set of cpu hotplug callback handlers to start
using the newly added cpu (see the set of callbacks in kernel/sched.c)

So, now we have a race between our callback and the scheduler's callbacks.
("Placing" our callback appropriately in a safe position using priority
for notifiers doesn't appeal to me that much, since it looks like too much
hackery. It should probably be our last resort).

Regards,
Srivatsa S. Bhat