linux-kernel - Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing completions

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Message-ID: <54DCC690.1070103@linux.vnet.ibm.com>
Date:	Thu, 12 Feb 2015 20:58:16 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	torvalds@...ux-foundation.org, konrad.wilk@...cle.com,
	pbonzini@...hat.com, paulmck@...ux.vnet.ibm.com,
	waiman.long@...com, davej@...hat.com, oleg@...hat.com,
	x86@...nel.org, jeremy@...p.org, paul.gortmaker@...driver.com,
	ak@...ux.intel.com, jasowang@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, riel@...hat.com,
	borntraeger@...ibm.com, akpm@...ux-foundation.org,
	a.ryabinin@...sung.com, sasha.levin@...cle.com, dave@...olabs.net
Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing
 completions

On 02/12/2015 08:30 PM, Peter Zijlstra wrote:
> On Thu, Feb 12, 2015 at 05:17:27PM +0530, Raghavendra K T wrote:
[...]
>>
>> Linus suggested that we should not do any writes to lock after unlock(),
>> and we can move slowpath clearing to fastpath lock.
>>
>> So this patch implements the fix with:
>> 1. Moving slowpath flag to head (Oleg).
>> 2. use xadd to avoid read/write after unlock that checks the need for
>> unlock_kick (Linus).
>
> Maybe spend a few more words explaining these things; something like:
>
> Unlocked locks don't care about the slowpath flag; therefore we can keep
> it set after the last unlock, as long as we clear it again on the first
> (try)lock -- this removes the write after unlock.

Nit: I 'll reword a bit here since slowpath flag would result in
unnecessary kick but otherwise harmless IMO.
something like:
Unlocked locks don't care about the slowpath flag; therefore we can keep
it set after the last unlock, and clear it again on the first (try)lock.
-- this removes the write after unlock. note that keeping slowpath flag 
would result in unnecessary kicks.

> By moving the slowpath flag from the tail to the head ticket we avoid
> the need to access both the head and tail tickets on unlock.
>
> We can further avoid the need for a read-after-release by using xadd;
> the prev head value will include the slowpath flag and indicate if we
> need to do PV kicking of suspended spinners -- on modern chips xadd
> isn't (much) more expensive than an add + load.
>
> Its 'obvious' now, but maybe not so much after we've all not looked at
> this for a few months.
>

Absolutely correct. Thanks Peter for the detailed and very helpful
writeup.

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