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

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Message-ID: <CANN689FSxOz+0Cu7EG_yKD8ZE1OpT4kyT+ybLfXSqaifodJRpw@mail.gmail.com>
Date:	Tue, 19 Feb 2013 02:07:45 +0800
From:	Michel Lespinasse <walken@...gle.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	linux-doc@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org, mingo@...nel.org,
	linux-arch@...r.kernel.org, linux@....linux.org.uk,
	xiaoguangrong@...ux.vnet.ibm.com, wangyun@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, rusty@...tcorp.com.au,
	rostedt@...dmis.org, rjw@...k.pl, namhyung@...nel.org,
	tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
	netdev@...r.kernel.org, oleg@...hat.com,
	vincent.guittot@...aro.org, 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 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.

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 ?)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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