linux-kernel - Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

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Date:	Mon, 18 Feb 2013 23:44:41 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	linux-doc@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	namhyung@...nel.org, mingo@...nel.org, linux-arch@...r.kernel.org,
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	vincent.guittot@...aro.org, tglx@...utronix.de,
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	oleg@...hat.com, sbw@....edu, tj@...nel.org,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of
 Per-CPU Reader-Writer Locks

On 02/18/2013 11:37 PM, Michel Lespinasse wrote:
> On Tue, Feb 19, 2013 at 1:56 AM, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> On 02/18/2013 09:51 PM, Srivatsa S. Bhat wrote:
>>> On 02/18/2013 09:15 PM, Michel Lespinasse wrote:
>>>> I don't see anything preventing a race with the corresponding code in
>>>> percpu_write_unlock() that sets writer_signal back to false. Did I
>>>> miss something here ? It seems to me we don't have any guarantee that
>>>> all writer signals will be set to true at the end of the loop...
>>>
>>> Ah, thanks for pointing that out! IIRC Oleg had pointed this issue in the last
>>> version, but back then, I hadn't fully understood what he meant. Your
>>> explanation made it clear. I'll work on fixing this.
>>
>> We can fix this by using the simple patch (untested) shown below.
>> The alternative would be to acquire the rwlock for write, update the
>> ->writer_signal values, release the lock, wait for readers to switch,
>> again acquire the rwlock for write with interrupts disabled etc... which
>> makes it kinda messy, IMHO. So I prefer the simple version shown below.
> 
> Looks good.
> 
> Another alternative would be to make writer_signal an atomic integer
> instead of a bool. That way writers can increment it before locking
> and decrement it while unlocking.
> 

Yep, that would also do. But the spinlock version looks simpler - no need
to check if the atomic counter is non-zero, no need to explicitly spin in
a tight-loop etc.

> To reduce the number of atomic ops during writer lock/unlock, the
> writer_signal could also be a global read_mostly variable (I don't see
> any downsides to that compared to having it percpu - or is it because
> you wanted all the fastpath state to be in one single cacheline ?)
> 

Yes, we (Oleg and I) debated for a while about global vs percpu, and then
finally decided to go with percpu to have cache benefits.

Regards,
Srivatsa S. Bhat

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