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Date:   Tue, 29 May 2018 14:35:34 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc:     linux-kernel@vger.kernel.org, <linux-arch@vger.kernel.org>,
        <andrea.parri@amarulasolutions.com>, <will.deacon@arm.com>,
        <peterz@infradead.org>, <boqun.feng@gmail.com>,
        <npiggin@gmail.com>, <dhowells@redhat.com>, <j.alglave@ucl.ac.uk>,
        <luc.maranget@inria.fr>, <akiyks@gmail.com>, <mingo@kernel.org>,
        <torvalds@linux-foundation.org>, <roman.penyaev@profitbricks.com>
Subject: Re: LKMM litmus test for Roman Penyaev's rcu-rr
In-Reply-To: <20180528220811.GA5221@linux.vnet.ibm.com>
Message-ID: <Pine.LNX.4.44L0.1805291424440.1458-100000@iolanthe.rowland.org>
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On Mon, 28 May 2018, Paul E. McKenney wrote:

> Hello!
> 
> The litmus test below is a first attempt to model Roman's rcu-rr
> round-robin RCU-protected linked list.  His test code, which includes
> the algorithm under test, may be found here:
> 
> https://github.com/rouming/rcu-rr/blob/master/rcu-rr.c
> 
> The P0() process below roughly corresponds to remove_conn_from_arr(),
> with litmus-test variable "c" standing in for the per-CPU ppcpu_con.
> Similarly, P1() roughly corresponds to get_next_conn_rr().  It claims
> that the algorithm is safe, and also claims that it becomes unsafe if
> either synchronize_rcu() is removed.

This algorithm (the one in the litmus test; I haven't looked at Roman's
code) does seem valid.  In addition to removing either
synchronize_rcu(), interchanging the order of the stores in P0 (c
first, then w) would also invalidate it.

This is a little unusual in that c is written by more than one thread 
with no protection.  It works because the writes are all stores of a 
single pointer.

Why does the litmus test use smp_store_release() in three places?  
There doesn't seem to be any need; WRITE_ONCE() would be sufficient.

Alan

> Does this in fact realistically model Roman's algorithm?  Either way,
> is there a better approach?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> C C-RomanPenyaev-list-rcu-rr
> 
> {
> 	int *z=1; (* List: v->w->x->y->z. Noncircular, but long enough. *)
> 	int *y=z;
> 	int *x=y;
> 	int *w=x;
> 	int *v=w; (* List head is v. *)
> 	int *c=w; (* Cache, emulating ppcpu_con. *)
> }
> 
> P0(int *c, int *v, int *w, int *x, int *y)
> {
> 	rcu_assign_pointer(*w, y); /* Remove x from list. */
> 	synchronize_rcu();
> 	r1 = READ_ONCE(*c);
> 	if (r1 == x) {
> 		WRITE_ONCE(*c, 0); /* Invalidate cache. */
> 		synchronize_rcu();
> 	}
> 	smp_store_release(x, 0);  /* Emulate kfree(x). */
> }
> 
> P1(int *c, int *v)
> {
> 	rcu_read_lock();
> 	r1 = READ_ONCE(*c); /* Pick up cache. */
> 	if (r1 == 0) {
> 		r1 = READ_ONCE(*v); /* Cache empty, start from head. */
> 	}
> 	r2 = rcu_dereference(*r1); /* Advance to next element. */
> 	smp_store_release(c, r2); /* Update cache. */
> 	rcu_read_unlock();
> 
> 	/* And repeat. */
> 	rcu_read_lock();
> 	r3 = READ_ONCE(*c);
> 	if (r3 == 0) {
> 		r3 = READ_ONCE(*v);
> 	}
> 	r4 = rcu_dereference(*r3);
> 	smp_store_release(c, r4);
> 	rcu_read_unlock();
> }
> 
> locations [0:r1; 1:r1; 1:r3; c; v; w; x; y]
> exists (1:r1=0 \/ 1:r2=0 \/ 1:r3=0 \/ 1:r4=0) (* Better not be freed!!! *)