From: "Alex Bennée" <alex.bennee@linaro.org>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>,
qemu-ppc@nongnu.org, rth@twiddle.net, qemu-devel@nongnu.org,
benh@kernel.crashing.org
Subject: Re: [Qemu-devel] [PATCH RFC 3/4] target-ppc: use atomic_cmpxchg for ld/st reservation
Date: Wed, 07 Sep 2016 08:13:31 +0100 [thread overview]
Message-ID: <87d1kgyyg4.fsf@linaro.org> (raw)
In-Reply-To: <20160907053459.GM2780@voom.fritz.box>
David Gibson <david@gibson.dropbear.id.au> writes:
> On Wed, Sep 07, 2016 at 10:17:42AM +0530, Nikunj A Dadhania wrote:
>> David Gibson <david@gibson.dropbear.id.au> writes:
>>
>> > [ Unknown signature status ]
>> > On Fri, Sep 02, 2016 at 12:02:55PM +0530, Nikunj A Dadhania wrote:
>> >> Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
>> >
>> > This really needs a comment indicating that this implementation isn't
>> > strictly correct (although probably good enough in practice).
>>
>> Sure. And it also does not help if someone uses any store other than
>> store conditional, that isn't taken care.
>>
>> Assumption here is the locking primitives use load with reservation and
>> store conditional. And no other ld/st variant touch this memory.
>
> So, a) I don't think this really relies on that: an ordinary store
> (assuming it changes the value) will still get picked up the cmpxchg.
> Well.. at least after a suitable memory barrier. Matching memory
> models between emulated and host cpus is a whole other kettle of fish.
Have you seen Pranith's memory barrier patches?
>
> I think this does matter, IIRC a kernel spin unlock on ppc is a
> barrier + plain store, no load locked or store conditional.
>
>> > Specifically a racing store which happens to store the same value
>> > which was already in memory should clobber the reservation, but won't
>> > with this implementation.
>> >
>> > I had a long discussion at KVM Forum with Emilio Costa about this, in
>> > which I discovered just how hard it is to strictly implement
>> > store-conditional semantics in terms of anything else. So, this is
>> > probably a reasonable substitute, but we should note the fact that
>> > it's not 100%.
>>
>> I will update the commit log.
>>
>> Regards,
>> Nikunj
>>
--
Alex Bennée
next prev parent reply other threads:[~2016-09-07 7:13 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-02 6:32 [Qemu-devel] [PATCH RFC 0/4] Enable MTTCG on PowerPC Nikunj A Dadhania
2016-09-02 6:32 ` [Qemu-devel] [PATCH RFC 1/4] spapr-hcall: take iothread lock during handler call Nikunj A Dadhania
2016-09-02 8:53 ` Greg Kurz
2016-09-02 9:28 ` Nikunj A Dadhania
2016-09-02 9:57 ` Greg Kurz
2016-09-03 16:31 ` Nikunj A Dadhania
2016-09-02 10:06 ` Thomas Huth
2016-09-03 16:33 ` Nikunj A Dadhania
2016-09-02 6:32 ` [Qemu-devel] [PATCH RFC 2/4] target-ppc: with MTTCG report more threads Nikunj A Dadhania
2016-09-02 9:28 ` Greg Kurz
2016-09-02 9:34 ` Nikunj A Dadhania
2016-09-02 10:45 ` Greg Kurz
2016-09-03 16:34 ` Nikunj A Dadhania
2016-09-07 3:51 ` David Gibson
2016-09-07 4:41 ` Nikunj A Dadhania
2016-09-02 6:32 ` [Qemu-devel] [PATCH RFC 3/4] target-ppc: use atomic_cmpxchg for ld/st reservation Nikunj A Dadhania
2016-09-07 4:02 ` David Gibson
2016-09-07 4:47 ` Nikunj A Dadhania
2016-09-07 5:24 ` Benjamin Herrenschmidt
2016-09-07 8:42 ` Nikunj A Dadhania
2016-09-07 5:34 ` David Gibson
2016-09-07 7:13 ` Alex Bennée [this message]
2016-09-12 1:19 ` David Gibson
2016-09-12 8:39 ` Alex Bennée
2016-09-12 9:15 ` Benjamin Herrenschmidt
2016-09-02 6:32 ` [Qemu-devel] [PATCH RFC 4/4] target-ppc: flush tlb from all the cpu Nikunj A Dadhania
2016-09-02 7:22 ` Benjamin Herrenschmidt
2016-09-02 7:34 ` Nikunj A Dadhania
2016-09-04 17:00 ` Alex Bennée
2016-09-04 22:17 ` Benjamin Herrenschmidt
2016-09-05 0:10 ` Benjamin Herrenschmidt
2016-09-06 1:55 ` Nikunj A Dadhania
2016-09-06 3:05 ` Benjamin Herrenschmidt
2016-09-06 4:53 ` Nikunj A Dadhania
2016-09-06 5:30 ` Benjamin Herrenschmidt
2016-09-06 6:57 ` Nikunj A Dadhania
2016-09-02 6:43 ` [Qemu-devel] [PATCH RFC 0/4] Enable MTTCG on PowerPC Cédric Le Goater
2016-09-02 6:46 ` Nikunj A Dadhania
2016-09-02 7:57 ` Thomas Huth
2016-09-02 11:44 ` Cédric Le Goater
2016-09-02 7:19 ` Benjamin Herrenschmidt
2016-09-02 7:39 ` Nikunj A Dadhania
2016-09-02 12:13 ` Benjamin Herrenschmidt
2016-09-07 4:08 ` David Gibson
2016-09-07 4:48 ` Nikunj A Dadhania
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