linux-kernel - Re: [PATCH] mm: slub: Ensure that slab

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Message-ID: <56E10B59.1060700@synopsys.com>
Date:	Thu, 10 Mar 2016 11:21:21 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	<linux-parisc@...r.kernel>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Helge Deller <deller@....de>, <linux-kernel@...r.kernel.org>,
	<stable@...r.kernel.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Pekka Enberg <penberg@...nel.org>, <linux-mm@...ck.org>,
	Noam Camus <noamc@...hip.com>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	<linux-snps-arc@...ts.infradead.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

On Wednesday 09 March 2016 08:21 PM, Peter Zijlstra wrote:
>> But in SLUB: bit_spin_lock() + __bit_spin_unlock() is acceptable ? How so
>> (ignoring the performance thing for discussion sake, which is a side effect of
>> this implementation).
> 
> The sort answer is: Per definition. They are defined to work together,
> which is what makes __clear_bit_unlock() such a special function.
> 
>> So despite the comment below in bit_spinlock.h I don't quite comprehend how this
>> is allowable. And if say, by deduction, this is fine for LLSC or lock prefixed
>> cases, then isn't this true in general for lot more cases in kernel, i.e. pairing
>> atomic lock with non-atomic unlock ? I'm missing something !
> 
> x86 (and others) do in fact use non-atomic instructions for
> spin_unlock(). But as this is all arch specific, we can make these
> assumptions. Its just that generic code cannot rely on it.

OK despite being obvious now, I was not seeing the similarity between spin_*lock()
and bit_spin*lock() :-(

ARC also uses standard ST for spin_unlock() so by analogy __bit_spin_unlock() (for
LLSC case) would be correctly paired with bit_spin_lock().

But then why would anyone need bit_spin_unlock() at all. Specially after this
patch from you which tightens __bit_spin_lock() even more for the general case.

Thing is if the API exists majority of people would would use the more
conservative version w/o understand all these nuances. Can we pursue the path of
moving bit_spin_unlock() over to __bit_spin_lock(): first changing the backend
only and if proven stable replacing the call-sites themselves.

> 
> So let me try and explain.
> 
> 
> The problem as identified is:
> 
> CPU0						CPU1
> 
> bit_spin_lock()					__bit_spin_unlock()
> 1:
> 	/* fetch_or, r1 holds the old value */
> 	spin_lock
> 	load	r1, addr
> 						load	r1, addr
> 						bclr	r2, r1, 1
> 						store	r2, addr
> 	or	r2, r1, 1
> 	store	r2, addr	/* lost the store from CPU1 */
> 	spin_unlock
> 
> 	and	r1, 1
> 	bnz	2	/* it was set, go wait */
> 	ret
> 
> 2:
> 	load	r1, addr
> 	and	r1, 1
> 	bnz	2	/* wait until its not set */
> 
> 	b	1	/* try again */
> 
> 
> 
> For LL/SC we replace:
> 
> 	spin_lock
> 	load	r1, addr
> 
> 	...
> 
> 	store	r2, addr
> 	spin_unlock
> 
> With the (obvious):
> 
> 1:
> 	load-locked	r1, addr
> 
> 	...
> 
> 	store-cond	r2, addr
> 	bnz		1 /* or whatever branch instruction is required to retry */
> 
> 
> In this case the failure cannot happen, because the store from CPU1
> would have invalidated the lock from CPU0 and caused the
> store-cond to fail and retry the loop, observing the new value. 

You did it again, A picture is worth thousand words !

Thx,
-Vineet