linux-kernel - Re: [PATCH v2 3/4] rcu/kvfree: Move need_offload

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Date:   Wed, 30 Nov 2022 13:56:17 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.iitr10@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v2 3/4] rcu/kvfree: Move need_offload_krc() out of
 krcp->lock

On Tue, Nov 29, 2022 at 03:38:33PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 29, 2022 at 04:58:21PM +0100, Uladzislau Rezki (Sony) wrote:
> > Currently a need_offload_krc() function requires the krcp->lock
> > to be held because krcp->head can not be checked concurrently.
> > 
> > Fix it by updating the krcp->head using WRITE_ONCE() macro so
> > it becomes lock-free and safe for readers to see a valid data
> > without any locking.
> 
> Don't we also need to use READ_ONCE() for the code loading this krcp->head
> pointer?  Or do the remaining plain C-language accesses somehow avoid
> running concurrently with those new WRITE_ONCE() invocations?
>
It can be concurrent. I was thinking about it. For some reason i decided
to keep readers as a "regular" ones for loading the krcp->head.

In this case it might take time for readers to see an updated value
as a worst case scenario.

So i need to update it or upload one more patch on top of v2. Should
i upload a new patch?

Thanks!

--
Uladzislau Rezki