Re: [PATCH 5/5] mm/slub: Refactor deactivate_slab()

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Fri, Feb 25, 2022 at 11:07:45AM +0100, Vlastimil Babka wrote:
> On 2/25/22 10:50, Hyeonggon Yoo wrote:
> > On Fri, Feb 25, 2022 at 09:34:09AM +0000, Hyeonggon Yoo wrote:
> >> On Thu, Feb 24, 2022 at 07:16:11PM +0100, Vlastimil Babka wrote:
> >> > On 2/21/22 11:53, Hyeonggon Yoo wrote:
> >> > > Simply deactivate_slab() by removing variable 'lock' and replacing
> >> > > 'l' and 'm' with 'mode'. Instead, remove slab from list and unlock
> >> > > n->list_lock when cmpxchg_double() fails, and then retry.
> >> > > 
> >> > > One slight functional change is releasing and taking n->list_lock again
> >> > > when cmpxchg_double() fails. This is not harmful because SLUB avoids
> >> > > deactivating slabs as much as possible.
> >> > > 
> >> > > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> >> > 
> >> > Hm I wonder if we could simplify even a bit more. Do we have to actually
> >> > place the slab on a partial (full) list before the cmpxchg, only to remove
> >> > it when cmpxchg fails? Seems it's to avoid anyone else seeing the slab
> >> > un-frozen, but not on the list, which would be unexpected. However if anyone
> >> > sees such slab, they have to take the list_lock first to start working with
> >> > the slab... so this should be safe, because we hold the list_lock here, and
> >> > will place the slab on the list before we release it. But it thus shouldn't
> >> > matter if the placement happens before or after a successful cmpxchg, no? So
> >> > we can only do it once after a successful cmpxchg and need no undo's?
> >> >
> >> 
> >> My thought was similar. But after testing I noticed that &n->list_lock prevents
> >> race between __slab_free() and deactivate_slab().
> >> 
> >> > Specifically AFAIK the only possible race should be with a __slab_free()
> >> > which might observe !was_frozen after we succeed an unfreezing cmpxchg and
> >> > go through the
> >> > "} else { /* Needs to be taken off a list */"
> >> > branch but then it takes the list_lock as the first thing, so will be able
> >> > to proceed only after the slab is actually on the list.
> >> > 
> >> > Do I miss anything or would you agree?
> >> >
> >> 
> >> It's so tricky.
> >> 
> >> I tried to simplify more as you said. Seeing frozen slab on list was not
> >> problem. But the problem was that something might interfere between
> >> cmpxchg_double() and taking spinlock.
> >> 
> >> This is what I faced:
> >> 
> >> 	CPU A				CPU B
> >> deactivate_slab() {			__slab_free() {
> >> 	/* slab is full */
> >> 	slab.frozen = 0;
> >> 	cmpxchg_double();
> >> 						/* Hmm... 
> >> 						slab->frozen == 0 &&
> >> 						slab->freelist != NULL?
> >> 						Oh This must be on the list.. */
> > 						Oh this is wrong.
> > 						slab->freelist must be
> > 						NULL because it's full
> > 						slab.
> > 
> > 						It's more complex
> > 						than I thought...
> > 
> > 
> >> 						spin_lock_irqsave();
> >> 						cmpxchg_double();
> >> 						/* Corruption: slab
> >> 						 * was not yet inserted to
> >> 						 * list but try removing */
> >> 						remove_full();
> >> 						spin_unlock_irqrestore();
> >> 					}
> >> 	spin_lock_irqsave();
> >> 	add_full();
> >> 	spin_unlock_irqrestore();
> >> }
> > 
> > So it was...
> > 
> >  	CPU A				CPU B
> >  deactivate_slab() {			__slab_free() {
> >  	/* slab is full */
> >  	slab.frozen = 0;
> >  	cmpxchg_double();
> >  						/*
> > 							Hmm... 
> > 							!was_frozen &&
> > 							prior == NULL?
> > 							Let's freeze this!
> > 						*/
> > 						put_cpu_partial();
> >  					}
> >  	spin_lock_irqsave();
> 
> Yeah in my proposal I didn't intend to only take spin_lock_irqsave() here.
> My idea for CPU A would be something like:
>
Oh, misunderstood your proposal.
I spent hours figuring what's wrong haha

> spin_lock_irqsave();
> slab.frozen = 0;
> if (cmpxchg_double()); {
> 	/* success */
> 	add_partial(); // (or add_full())
> 	spin_unlock_irqrestore();
> } else {
> 	/* fail */
> 	spin_unlock_irqrestore();
> 	goto redo;
> }
> 	
> So we would still have the list_lock protection around cmpxchg as in the
> current code. We just wouldn't do e.g. add_partial() before cmpxchg, only to
> remove_partial() when cmpxchg failed.

Now I got what you mean...
I think that would work. Will try that.

Thank you for nice proposal!

> 
> >  	add_full();
> > 	/* It's now frozen by CPU B and at the same time on full list */
> >  	spin_unlock_irqrestore();
> > 
> > And &n->list_lock prevents such a race.
> 

-- 
Thank you, You are awesome!
Hyeonggon :-)