Messages in this thread | | | Date | Mon, 21 Jan 2019 12:45:05 +0100 | From | Andrea Parri <> | Subject | Re: [PATCH] kcov: convert kcov.refcount to refcount_t |
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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 <keescook@chromium.org> > > > Reviewed-by: David Windsor <dwindsor@gmail.com> > > > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com> > > > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> > > > --- > > > 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 <linux/debugfs.h> > > > #include <linux/uaccess.h> > > > #include <linux/kcov.h> > > > +#include <linux/refcount.h> > > > #include <asm/setup.h> > > > > > > /* 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 > > >
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