linux-kernel - Re: [PATCH 01/18] drivers core: Add I/O ASID allocator

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Message-Id: <20190409145308.GZ14111@linux.ibm.com>
Date:   Tue, 9 Apr 2019 07:53:08 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH 01/18] drivers core: Add I/O ASID allocator

On Tue, Apr 09, 2019 at 01:30:30PM +0300, Andriy Shevchenko wrote:
> On Tue, Apr 09, 2019 at 03:04:36AM -0700, Christoph Hellwig wrote:
> > On Tue, Apr 09, 2019 at 01:00:49PM +0300, Andriy Shevchenko wrote:
> > > I think it makes sense to add a helper macro to rcupdate.h
> > > (and we have several cases in kernel that can utilize it)
> > > 
> > > #define kfree_non_null_rcu(ptr, rcu_head)		\
> > > 	do {						\
> > > 		if (ptr)				\
> > > 			kfree_rcu(ptr, rcu_head);	\
> > > 	} while (0)
> > > 
> > > as a more common pattern for resource deallocators.
> > 
> > I think that should move straight into kfree_rcu.  
> 
> Possible. I didn't dare to offer this due to lack of knowledge how it's used in
> other places.
> 
> > In general
> > we expect *free* to deal with NULL pointers transparently, so we
> > should do so here as well.
> 
> Exactly my point, thanks.

As shown below?

And now that you mention it, it is a bit surprising that no one has
complained before.  ;-)

							Thanx, Paul

------------------------------------------------------------------------

commit 23ad938244968e9d2a8001a1c52887c113b182f6
Author: Paul E. McKenney <paulmck@...ux.ibm.com>
Date:   Tue Apr 9 07:48:18 2019 -0700

    rcu: Make kfree_rcu() ignore NULL pointers
    
    This commit makes the kfree_rcu() macro's semantics be consistent
    with the likes of kfree() by adding a check for NULL pointers, so
    that kfree_rcu(NULL, ...) is a no-op.
    
    Reported-by: Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>
    Reported-by: Christoph Hellwig <hch@...radead.org>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 922bb6848813..c68649b9bcec 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -828,9 +828,13 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
  * The BUILD_BUG_ON check must not involve any function calls, hence the
  * checks are done in macros here.
  */
-#define kfree_rcu(ptr, rcu_head)					\
-	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
-
+#define kfree_rcu(ptr, rhf)						\
+do {									\
+	typeof (ptr) ___p = (ptr);					\
+									\
+	if (___p)							\
+		__kfree_rcu(&((___p)->rhf), offsetof(typeof(*(ptr)), rhf)); \
+} while (0)
 
 /*
  * Place this after a lock-acquisition primitive to guarantee that