Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Waiman Long
Date: Fri Jun 14 2013 - 14:17:29 EST

On 06/14/2013 11:37 AM, Linus Torvalds wrote:
On Fri, Jun 14, 2013 at 8:00 AM, Waiman Long<waiman.long@xxxxxx> wrote:
On 06/12/2013 08:59 PM, Linus Torvalds wrote:
Ho humm.. interesting. I was talking about wanting to mix atomics and
spinlocks earlier in this thread due to space constraints, and it
strikes me that that would actually help this case a lot. Having the
dentry count mix d_lock and the count in one word would allow for
atomic ops like "increment if not locked", and we'd avoid this whole
race entirely..

Something like "low bit of count is the lock bit" would end up being
lovely for this case. Of course, that's not how our spinlocks work ..


I have created another patch to do exactly the "increment if not locked"
operation as suggested. It did help a lot. See the patch below for more
information. Any additional comment will be appreciated.
Hmm. This is interesting and proves the concept, and the numbers look
very promising.

The patch is not mergable, though, since it clearly depends on the
spinlock/d_count fitting in a u64, which is normally true, but not the
case of debugging locks etc, we'd need to generalize and fix the whole
concept of "refcount+lock".

With some minor changes, the current patch can be modified to support debugging lock for 32-bit system. For 64-bit system, we can apply a similar concept for debugging lock with cmpxchg_double. However, for architecture that does not have cmpxchg_double support, it will be out of luck and we probably couldn't support the same feature in debugging mode. It will have to fall back to taking the lock.

I was thinking about generalizing the fix, but one issue that I was aware of was that the d_lock member of dentry had more than 300 references throughout the filesystem code. A general fix will require d_lock to be accessed in a different way. So it will be a pretty massive patch touching quite a lot of files even though the changes will be pretty straightforward in most cases.

Generalizing it might be a good idea anyway, since there are other
cases of "atomic_dec_and_lock()" etc behaviours where we might want to
have these kinds of extended lock+count shenanigans.

The patch can certainly be generalized. I will see what I can do in a week of two.

I also do wonder if we could perhaps fit both in 32-bits, and just not
use the "real" spinlocks at all, but use a bitlock in the low (or
high) bit of the refcount. We do that in some other places - we'd
potentially lose lockdep etc, and we'd lose some of the other good
parts of spinlocks (fairness yadda yadda), but *if* we can reduce
contention enough that it works out, maybe it would be worth it.

As the dentry is such an important data structure for the filesystem layer, losing the fairness attribute and the ability to debug may be too high a price to pay. For other niche cases, such a combined data type can certainly be used.

So this doesn't look like 3.11 material, but the numbers certainly
make it look very promising, so with some more work on it ...


When will be the deadline for stable patch that can be considered to be merged in 3.11?

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