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[109.80.100.85]) by smtp.gmail.com with ESMTPSA id p1-v6sm4663904ejo.49.2019.01.21.03.45.11 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 21 Jan 2019 03:45:11 -0800 (PST) Date: Mon, 21 Jan 2019 12:45:05 +0100 From: Andrea Parri To: Dmitry Vyukov Cc: Elena Reshetova , Kees Cook , Peter Zijlstra , Alan Stern , "Paul E. McKenney" , Will Deacon , Andrew Morton , Andrey Ryabinin , Anders Roxell , Mark Rutland , LKML Subject: Re: [PATCH] kcov: convert kcov.refcount to refcount_t Message-ID: <20190121114505.GA7307@andrea> References: <1547634429-772-1-git-send-email-elena.reshetova@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 21, 2019 at 10:52:37AM +0100, Dmitry Vyukov wrote: [...] > > Am I missing something or refcount_dec_and_test does not in fact > > provide ACQUIRE ordering? > > > > +case 5) - decrement-based RMW ops that return a value > > +----------------------------------------------------- > > + > > +Function changes: > > + atomic_dec_and_test() --> refcount_dec_and_test() > > + atomic_sub_and_test() --> refcount_sub_and_test() > > + no atomic counterpart --> refcount_dec_if_one() > > + atomic_add_unless(&var, -1, 1) --> refcount_dec_not_one(&var) > > + > > +Memory ordering guarantees changes: > > + fully ordered --> RELEASE ordering + control dependency > > > > I think that's against the expected refcount guarantees. When I > > privatize an atomic_dec_and_test I would expect that not only stores, > > but also loads act on a quiescent object. But loads can hoist outside > > of the control dependency. > > > > Consider the following example, is it the case that the BUG_ON can still fire? Can't it fire in an SC world? (wrong example, or a Monday morning? ;D) > > > > struct X { > > refcount_t rc; // == 2 > > int done1, done2; // == 0 > > }; > > > > // thread 1: > > x->done1 = 1; > > if (refcount_dec_and_test(&x->rc)) > > BUG_ON(!x->done2); > > > > // thread 2: > > x->done2 = 1; > > if (refcount_dec_and_test(&x->rc)) > > BUG_ON(!x->done1); > > +more people knowledgeable in memory ordering > > Unfortunately I can't find a way to reply to the > Documentation/core-api/refcount-vs-atomic.rst patch review thread. > > The refcount_dec_and_test guarantees look too weak to me, see the > example above. Shouldn't refcount_dec_and_test returning true give the > object in fully quiescent state? Why only control dependency? Loads > can hoist across control dependency, no? As you remarked, the doc. says CTRL+RELEASE (so yes, loads can hoist); AFAICR, implementations do comply to this requirement. (FWIW, I sometimes think at this "weird" ordering as a weak "acq_rel", the latter, acq_rel, being missing from the current APIs.) Andrea > > > > > > Suggested-by: Kees Cook > > > Reviewed-by: David Windsor > > > Reviewed-by: Hans Liljestrand > > > Signed-off-by: Elena Reshetova > > > --- > > > kernel/kcov.c | 9 +++++---- > > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > > > diff --git a/kernel/kcov.c b/kernel/kcov.c > > > index c2277db..051e86e 100644 > > > --- a/kernel/kcov.c > > > +++ b/kernel/kcov.c > > > @@ -20,6 +20,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > > > > /* Number of 64-bit words written per one comparison: */ > > > @@ -44,7 +45,7 @@ struct kcov { > > > * - opened file descriptor > > > * - task with enabled coverage (we can't unwire it from another task) > > > */ > > > - atomic_t refcount; > > > + refcount_t refcount; > > > /* The lock protects mode, size, area and t. */ > > > spinlock_t lock; > > > enum kcov_mode mode; > > > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch); > > > > > > static void kcov_get(struct kcov *kcov) > > > { > > > - atomic_inc(&kcov->refcount); > > > + refcount_inc(&kcov->refcount); > > > } > > > > > > static void kcov_put(struct kcov *kcov) > > > { > > > - if (atomic_dec_and_test(&kcov->refcount)) { > > > + if (refcount_dec_and_test(&kcov->refcount)) { > > > vfree(kcov->area); > > > kfree(kcov); > > > } > > > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep) > > > if (!kcov) > > > return -ENOMEM; > > > kcov->mode = KCOV_MODE_DISABLED; > > > - atomic_set(&kcov->refcount, 1); > > > + refcount_set(&kcov->refcount, 1); > > > spin_lock_init(&kcov->lock); > > > filep->private_data = kcov; > > > return nonseekable_open(inode, filep); > > > -- > > > 2.7.4 > > >