linux-kernel - Re: Avoiding the dentry d_lock on final dput(), part deux: transactional memory

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Message-ID: <CAEjGV6xuscOh=t6BhbdRKAPcZ1DOJkD6ZcB1wgTf9-AedKSaxA@mail.gmail.com>
Date:	Tue, 1 Oct 2013 14:52:28 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: Avoiding the dentry d_lock on final dput(), part deux:
 transactional memory

>> Well we don't have to, I think Mikey wasn't totally clear about that
>> "making all registers volatile" business :-) This is just something we
>> need to handle in assembly if we are going to reclaim the suspended
>> transaction.

Yeah, sorry.  The slow path with all registers as volatile is only
needed if we get pre-empted during the transaction.

>>
>> So basically, what we need is something along the lines of
>> enable_kernel_tm() which checks if there's a suspended user transaction
>> and if yes, kills/reclaims it.
>>
>> Then we also need to handle in our interrupt handlers that we have an
>> active/suspended transaction from a kernel state, which we don't deal
>> with at this point, and do whatever has to be done to kill it... we
>> might get away with something simple if we can state that we only allow
>> kernel transactions at task level and not from interrupt/softirq
>> contexts, at least initially.
>
> Call me a coward, but this is starting to sound a bit scary.  ;-)

We are just wanting to prototype it for now to see if we could make it
go faster.  If it's worth it, then we'd consider the additional
complexity this would bring.

I don't think it'll be that bad, but I'd certainly want to make sure
it's worth it before trying :-)

Mikey
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