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SubjectRe: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs
On 12/20/2011 11:29 PM, Al Viro wrote:

> On Tue, Dec 20, 2011 at 08:05:32PM +0530, Srivatsa S. Bhat wrote:
>> Sorry but I didn't quite get your point...
>> No two cpu hotplug operations can race because of the cpu_hotplug lock they
>> use. Hence, if a cpu online operation begins, it has to succeed or fail
>> eventually. No other cpu hotplug operation can intervene. Ditto for cpu offline
>> operations.
>> Hence a CPU_UP_PREPARE event *will* be followed by a corresponding
>> CPU_UP_CANCELED or CPU_ONLINE event for the same cpu. (And we ignore the
>> CPU_STARTING event that comes in between, on purpose, so as to avoid the race
>> with cpu_online_mask). Similar is the story for offline operation.
>> And if the notifier grabs the spinlock and keeps it locked between these 2
>> points of a cpu hotplug operation, it ensures that our br locks will spin,
>> instead of block till the cpu hotplug operation is complete. Isn't this what
>> we desired all along? "A non-blocking way to sync br locks with cpu hotplug"?
>> Or am I missing something?
> The standard reason why losing the timeslice while holding a spinlock means
> deadlocks?
> CPU1: grabs spinlock
> CPU[2..n]: tries to grab the same spinlock, spins
> CPU1: does something blocking, process loses timeslice
> CPU1: whatever got scheduled there happens to to try and grab the same
> spinlock and you are stuck. At that point *all* CPUs are spinning on
> that spinlock and your code that would eventually unlock it has no chance
> to get any CPU to run on.
> Having the callback grab and release a spinlock is fine (as long as you
> don't do anything blocking between these spin_lock/spin_unlock). Having
> it leave with spinlock held, though, means that the area where you can't
> block has expanded a whole lot. As I said, brittle...

Ah, now I see your point! Thanks for the explanation.

> A quick grep through the actual callbacks immediately shows e.g.
> del_timer_sync() done on CPU_DOWN_PREPARE. And sysfs_remove_group(),
> which leads to outright mutex_lock(). And sysfs_remove_link() (ditto).
> And via_cputemp_device_remove() (again, mutex_lock()). And free_irq().
> And perf_event_exit_cpu() (mutex_lock()). And...
> IOW, there are shitloads of deadlocks right there. If your callback's
> position in the chain is earlier than any of those, you are screwed.

The thought makes me shudder!

> No, what I had in mind was different - use the callbacks to maintain a

> bitmap that would contain
> a) all CPUs that were online at the moment
> b) ... and not too much else
> Updates protected by spinlock; in all cases it gets dropped before the
> callback returns. br_write_lock() grabs that spinlock and iterates over
> the set; it *does* leave the spinlock grabbed - that's OK, since all
> code between br_write_lock() and br_write_unlock() must be non-blocking
> anyway. br_write_unlock() iterates over the same bitmap (unchanged since
> br_write_lock()) and finally drops the spinlock.

I had this same thing in mind when I started out to write the patch.. but
after Cong raised that concern, I changed track and in the meantime, tried
to get rid of maintaining our own bitmap as well...
But unfortunately that turned out to be disastrous!

> AFAICS, what we want in callback is
> grab spinlock
> remove cpu from bitmap
> drop spinlock
> grab spinlock
> add cpu to bitmap
> drop spinlock
> That ought to keep bitmap close to cpu_online_mask, which is enough for
> our purposes.

Yes, that should do. And while initializing our bitmap, we could use

make a copy of cpu_online_mask

since blocking here is acceptable, as this is done in the lock_init() code.

That would be better than

grab spinlock
add N to bitmap
release spinlock

because the latter code is not fully race-free (because we don't handle
CPU_DOWN_PREPARE event in the callback and hence cpu_online_mask can get
updated in-between). But it would still work since cpus going down don't
really pose problems for us.

However the former code is race-free, and we can afford it since we are
free to block at that point.

Srivatsa S. Bhat

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