linux-kernel - Re: [PATCH] SRCU: More efficient reader counts.

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Date:   Mon, 23 Jan 2017 13:33:59 -0800
From:   Lance Roy <ldr709@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
        bobby.prani@...il.com
Subject: Re: [PATCH] SRCU: More efficient reader counts.

On Mon, 23 Jan 2017 12:35:08 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Mon, Jan 23, 2017 at 12:17:25PM -0800, Lance Roy wrote:
> > SRCU uses two per-cpu counters: a nesting counter to count the number of
> > active critical sections, and a sequence counter to ensure that the nesting
> > counters don't change while they are being added together in
> > srcu_readers_active_idx_check().
> > 
> > This patch instead uses per-cpu lock and unlock counters. Because the both
> > counters only increase and srcu_readers_active_idx_check() reads the unlock
> > counter before the lock counter, this achieves the same end without having
> > to increment two different counters in srcu_read_lock(). This also saves a
> > smp_mb() in srcu_readers_active_idx_check().
> > 
> > Possible bug: There is no guarantee that the lock counter won't overflow
> > during srcu_readers_active_idx_check(), as there are no memory barriers
> > around srcu_flip() (see comment in srcu_readers_active_idx_check() for
> > details). However, this problem was already present before this patch.
> > 
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Signed-off-by: Lance Roy <ldr709@...il.com>  
> 
> In general, the comment update looks good, but this patch undoes my
> application of review feedback to your original patch.  Could you please
> submit the comment update as a separate patch on top of branch rcu/next
> of my -rcu tree?
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
I have added the review feedback to the new patch. I will send the new version.
If you prefer that the comment changes are in a separate patch I can change it,
but I thought it would be better not to have an incorrect statement in the
commit message.

> Of course, if any of the changes from the review feedback are in any
> way problematic, please do let me know.
I don't see any problems.

Thanks,
Lance