Re: [GIT PULL rcu/next] RCU commits for 4.13

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jun 14, 2017 at 04:33:22PM +0200, Andrea Parri wrote:
> On Tue, Jun 13, 2017 at 09:33:17PM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 14, 2017 at 04:54:04AM +0200, Andrea Parri wrote:
> > > On Mon, Jun 12, 2017 at 02:37:55PM -0700, Paul E. McKenney wrote:
> > > > Hello, Ingo,
> > > > 
> > > > This pull request is unusual in being a single linear set of commits,
> > > > as opposed to my usual topic branches.  This is due to the many
> > > > large-footprint changes, which means that reasonable topic branches
> > > > result in large numbers of merge conflicts.  In addition, some commits
> > > > depend on other commits that should be on different topic branches.
> > > > I will return to the topic-branch style next time.
> > > > 
> > > > The largest feature of this series is shrinking and simplification,
> > > > with the following diffstat summary:
> > > > 
> > > >  79 files changed, 1496 insertions(+), 4211 deletions(-)
> > > > 
> > > > In other words, this series represents a net reduction of more than 2700
> > > > lines of code.
> > > > 
> > > > These commits were posted to LKML:
> > > > 
> > > > 	http://lkml.kernel.org/r/20170525215934.GA11578@linux.vnet.ibm.com
> > > 
> > > I did raise some issues (AFAICT, unresolved) concerning...
> > > 
> > > 
> > > > 
> > > > Two of these commits (46/88 and 48/88) have been deferred, most likely
> > > > to v4.14.  All of the remaining commits have been subjected to the 0day
> > > > Test Robot and -next testing, and are availiable in teh git repository at:
> > > > 
> > > >   git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git for-mingo
> > > > 
> > > > for you to fetch changes up to 6d48152eafde1f0d0a4a9e0584fa7d9ff4fbfdac:
> > > > 
> > > >   rcu: Remove RCU CPU stall warnings from Tiny RCU (2017-06-08 18:52:45 -0700)
> > > > 
> > > > ----------------------------------------------------------------
> > > > Arnd Bergmann (1):
> > > >       bcm47xx: Fix build regression
> > > > 
> > > > Paul E. McKenney (83):
> > > >       rcutorture: Add lockdep to one of the SRCU scenarios
> > > >       rcutorture: Add three-level tree test for Tree SRCU
> > > >       rcutorture: Fix bug in reporting Kconfig mis-settings
> > > >       rcutorture: Add a scenario for Tiny SRCU
> > > >       rcutorture: Add a scenario for Classic SRCU
> > > >       rcu: Prevent rcu_barrier() from starting needless grace periods
> > > >       rcutorture: Correctly handle CONFIG_RCU_TORTURE_TEST_* options
> > > >       rcutorture: Update test scenarios based on new Kconfig dependencies
> > > >       srcu: Eliminate possibility of destructive counter overflow
> > > >       rcu: Complain if blocking in preemptible RCU read-side critical section
> > > >       rcuperf: Defer expedited/normal check to end of test
> > > >       rcuperf: Remove conflicting Kconfig options
> > > >       rcu: Remove obsolete reference to synchronize_kernel()
> > > >       rcuperf: Add ability to performance-test call_rcu() and friends
> > > >       rcuperf: Add a Kconfig-fragment file for Classic SRCU
> > > >       rcu: Make sync_rcu_preempt_exp_done() return bool
> > > >       checkpatch: Remove checks for expedited grace periods
> > > >       rcuperf: Add test for dynamically initialized srcu_struct
> > > >       doc/atomic_ops: Clarify smp_mb__{before,after}_atomic()
> > > >       atomics: Add header comment so spin_unlock_wait()
> > > 
> > > ... this one: c.f.,
> > > 
> > >   http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1418503.html
> > > 
> > > Any hints about those?
> > 
> > I suggest being -extremely- clear.  This is about ARM, correct?
> > If so, lay out the exact situation (code example, which hardware,
> > which spin_unlock_wait(), which sequence of events) that could lead to
> > the failure.
> > 
> > The problem here is that no one knows which of the 30 CPU families you
> > might be talking about, nor do they know exactly what the problem is.
> > I didn't worry about it at the time because I figured that you had
> > sent private email to Will with the full story.
> > 
> > Yes, the four of us (you, Alan, Luc, and me) discussed it, but we weren't
> > sure whether it was a bug in the memory model, the spin_unlock_wait()
> > code, or my description of spin_unlock_wait().  Given that Will didn't
> > object to my April 13th email (the one that you were not CCed on),
> > I figured that he wasn't going to claim that the spin_unlock_wait()
> > description was wrong, especially since he went to so much effort some
> > years back to make ARM64 meet that description.
> > 
> > So again, I recommend replying to your msg1418503.html email with
> > a code fragment demonstrating the problem, exact identification of
> > the hardware that might be susceptible (ARM64? ARM32? Which ARM32?),
> > exact identification of which spin_unlock_wait() function you suspect,
> > and a clear bullet-form sequence of events that shows how you believe
> > that the problem can occur.
> > 
> > That makes it easy for people to see what your concern is, easy for
> > them to check their code and hardware, and hard for them to ignore you.
> > 
> > Make sense?
> 
> My concerns originates from the fact that none of the implementations
> (of spin_unlock_wait()) for the architectures touched by:
> 
>   726328d92a42b6d4b76078e2659f43067f82c4e8
>   ("locking/spinlock, arch: Update and fix spin_unlock_wait() implementations"
> 
> currently contain any traces of that RELEASE/spin_unlock() from your:
> 
>   "Semantically this is equivalent to a spin_lock() immediately followed
>    by a spin_unlock()."
> 
> In fact, the header of that commit states:
> 
>   "The update is in semantics; where it previously was only a control
>    dependency, we now upgrade to a full load-acquire [...]"
> 
> For an example leveraging this RELEASE, consider:
> 
>   [initially: X = 0, s UNLOCKED]
> 
>   P0                      P1
>   X = 1;                  spin_lock(s);
>   spin_unlock_wait(s);    r0 = X;
> 
> According to the "spin_lock(); spin_unlock() semantics" this has one
> non-deadlocking execution, and the RELEASE from the spin_unlock_wait()
> (paired with the ACQUIRE from the spin_lock() in P1) guarantees that
> r0 = 1 in this execution. AFAICT, this same conclusion does not hold
> according to the "smp_cond_load_acquire() semantics" (726328d92a42b).

OK.  For exactly which CPU families do you believe that this fails to
hold.  That is, given the implementations of spin_unlock_wait() and
spin_lock() for the various CPU families, which will break and why?

							Thanx, Paul