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McKenney" , Peter Zijlstra , Will Deacon Subject: Re: [PATCH 1/3] LKMM: Add litmus test for RCU GP guarantee where updater frees object Message-ID: <20200320102603.GA22784@andrea> References: <20200320065552.253696-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200320065552.253696-1-joel@joelfernandes.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 20, 2020 at 02:55:50AM -0400, Joel Fernandes (Google) wrote: > This adds an example for the important RCU grace period guarantee, which > shows an RCU reader can never span a grace period. > > Signed-off-by: Joel Fernandes (Google) > --- > .../litmus-tests/RCU+sync+free.litmus | 40 +++++++++++++++++++ > 1 file changed, 40 insertions(+) > create mode 100644 tools/memory-model/litmus-tests/RCU+sync+free.litmus > > diff --git a/tools/memory-model/litmus-tests/RCU+sync+free.litmus b/tools/memory-model/litmus-tests/RCU+sync+free.litmus > new file mode 100644 > index 0000000000000..c4682502dd296 > --- /dev/null > +++ b/tools/memory-model/litmus-tests/RCU+sync+free.litmus > @@ -0,0 +1,40 @@ > +C RCU+sync+free > + > +(* > + * Result: Never > + * > + * This litmus test demonstrates that an RCU reader can never see a write after > + * the grace period, if it saw writes that happen before the grace period. This > + * is a typical pattern of RCU usage, where the write before the grace period > + * assigns a pointer, and the writes after destroy the object that the pointer > + * points to. > + * > + * This guarantee also implies, an RCU reader can never span a grace period and > + * is an important RCU grace period memory ordering guarantee. > + *) > + > +{ > +x = 1; > +y = x; > +z = 1; FYI, this could become a little more readable if we wrote it as follows: int x = 1; int *y = &x; int z = 1; The LKMM tools are happy either way, just a matter of style/preference; and yes, MP+onceassign+derefonce isn't currently following mine... ;-/ > +} > + > +P0(int *x, int *z, int **y) > +{ > + int r0; This would need to be "int *r0;" in order to make klitmus7(+gcc) happy. > + int r1; > + > + rcu_read_lock(); > + r0 = rcu_dereference(*y); > + r1 = READ_ONCE(*r0); > + rcu_read_unlock(); > +} > + > +P1(int *x, int *z, int **y) > +{ > + rcu_assign_pointer(*y, z); AFAICT, you don't need this "RELEASE"; e.g., compare this test with the example in: https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html#Grace-Period%20Guarantee What am I missing? Thanks, Andrea > + synchronize_rcu(); > + WRITE_ONCE(*x, 0); > +} > + > +exists (0:r0=x /\ 0:r1=0) > -- > 2.25.1.696.g5e7596f4ac-goog >