linux-kernel - Re: [PATCH 3/4] rculist: add list_for_each_entry_from

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Message-Id: <20180430153505.GW26088@linux.vnet.ibm.com>
Date:   Mon, 30 Apr 2018 08:35:05 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Josh Triplett <josh@...htriplett.org>, NeilBrown <neilb@...e.com>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] rculist: add list_for_each_entry_from_rcu()

On Mon, Apr 30, 2018 at 11:14:54AM -0400, Steven Rostedt wrote:
> On Mon, 30 Apr 2018 06:43:08 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Sun, Apr 29, 2018 at 10:20:33PM -0700, Josh Triplett wrote:
> > > On Mon, Apr 30, 2018 at 02:31:30PM +1000, NeilBrown wrote:  
> > > > list_for_each_entry_from_rcu() is an RCU version of
> > > > list_for_each_entry_from().  It walks a linked list under rcu
> > > > protection, from a given start point.
> > > > 
> > > > It is similar to list_for_each_entry_continue_rcu() but starts *at*
> > > > the given position rather than *after* it.
> > > > 
> > > > Naturally, the start point must be known to be in the list.  
> > > 
> > > I'd suggest giving an explicit advisory comment to clarify and suggest
> > > correct usage:
> > > 
> > > "This would typically require either that you obtained the node from a
> > > previous walk of the list in the same RCU read-side critical section, or
> > > that you held some sort of non-RCU reference (such as a reference count)
> > > to keep the node alive *and* in the list."
> > > 
> > > (Feel free to wordsmith the exact wording, but something like that seems
> > > like it would help people understand how to use this correctly, and make
> > > it less likely that they'd use it incorrectly.)  
> > 
> > What Josh said!  Could you also contrast this with the existing
> > list_for_each_entry_continue_rcu() macro in the header comment as well
> > as in the commit log?
> 
> Should Documentation/RCU/whatisRCU.txt be updated?
> 
> Specifically section: 7.  FULL LIST OF RCU APIs

Very much so, thank you for catching this!

							Thanx, Paul