[Xen-devel] [PATCH v2 4/4] xen/rcu: add assertions to debug build

From: Juergen Gross <jgross@suse.com>
To: xen-devel@lists.xenproject.org
Cc: Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: [Xen-devel] [PATCH v2 4/4] xen/rcu: add assertions to debug build
Date: Tue, 18 Feb 2020 13:21:14 +0100	[thread overview]
Message-ID: <20200218122114.17596-5-jgross@suse.com> (raw)
In-Reply-To: <20200218122114.17596-1-jgross@suse.com>

Xen's RCU implementation relies on no softirq handling taking place
while being in a RCU critical section. Add ASSERT()s in debug builds
in order to catch any violations.

For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
counter instead of preempt_[en|dis]able() as this enables to test
that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
usable there due to __cpu_up() calling process_pending_softirqs()
while holding the cpu hotplug lock).

Dropping the now no longer needed #include of preempt.h in rcupdate.h
requires adding it in some sources.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 xen/common/multicall.c     |  1 +
 xen/common/rcupdate.c      |  4 ++++
 xen/common/softirq.c       |  2 ++
 xen/common/wait.c          |  1 +
 xen/include/xen/rcupdate.h | 21 +++++++++++++++++----
 5 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/xen/common/multicall.c b/xen/common/multicall.c
index 5a199ebf8f..67f1a23485 100644
--- a/xen/common/multicall.c
+++ b/xen/common/multicall.c
@@ -10,6 +10,7 @@
 #include <xen/multicall.h>
 #include <xen/guest_access.h>
 #include <xen/perfc.h>
+#include <xen/preempt.h>
 #include <xen/trace.h>
 #include <asm/current.h>
 #include <asm/hardirq.h>
diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index e6add0b120..b03f4b44d9 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -46,6 +46,10 @@
 #include <xen/cpu.h>
 #include <xen/stop_machine.h>
 
+#ifndef NDEBUG
+DEFINE_PER_CPU(unsigned int, rcu_lock_cnt);
+#endif
+
 /* Global control variables for rcupdate callback mechanism. */
 static struct rcu_ctrlblk {
     long cur;           /* Current batch number.                      */
diff --git a/xen/common/softirq.c b/xen/common/softirq.c
index 3fe75ca3e8..18be8db0c6 100644
--- a/xen/common/softirq.c
+++ b/xen/common/softirq.c
@@ -30,6 +30,8 @@ static void __do_softirq(unsigned long ignore_mask, bool rcu_allowed)
     unsigned int i, cpu;
     unsigned long pending;
 
+    ASSERT(!rcu_allowed || rcu_quiesce_allowed());
+
     for ( ; ; )
     {
         /*
diff --git a/xen/common/wait.c b/xen/common/wait.c
index 24716e7676..9cdb174036 100644
--- a/xen/common/wait.c
+++ b/xen/common/wait.c
@@ -19,6 +19,7 @@
  * along with this program; If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <xen/preempt.h>
 #include <xen/sched.h>
 #include <xen/softirq.h>
 #include <xen/wait.h>
diff --git a/xen/include/xen/rcupdate.h b/xen/include/xen/rcupdate.h
index 87f35b7704..a5ee7fec2b 100644
--- a/xen/include/xen/rcupdate.h
+++ b/xen/include/xen/rcupdate.h
@@ -34,10 +34,23 @@
 #include <xen/cache.h>
 #include <xen/spinlock.h>
 #include <xen/cpumask.h>
-#include <xen/preempt.h>
+#include <xen/percpu.h>
 
 #define __rcu
 
+#ifndef NDEBUG
+DECLARE_PER_CPU(unsigned int, rcu_lock_cnt);
+
+#define rcu_quiesce_disable() (this_cpu(rcu_lock_cnt))++
+#define rcu_quiesce_enable()  (this_cpu(rcu_lock_cnt))--
+#define rcu_quiesce_allowed() (!this_cpu(rcu_lock_cnt))
+
+#else
+#define rcu_quiesce_disable() ((void)0)
+#define rcu_quiesce_enable()  ((void)0)
+#define rcu_quiesce_allowed() true
+#endif
+
 /**
  * struct rcu_head - callback structure for use with RCU
  * @next: next update requests in a list
@@ -90,16 +103,16 @@ typedef struct _rcu_read_lock rcu_read_lock_t;
  * will be deferred until the outermost RCU read-side critical section
  * completes.
  *
- * It is illegal to block while in an RCU read-side critical section.
+ * It is illegal to process softirqs while in an RCU read-side critical section.
  */
-#define rcu_read_lock(x)       ({ ((void)(x)); preempt_disable(); })
+#define rcu_read_lock(x)       ({ ((void)(x)); rcu_quiesce_disable(); })
 
 /**
  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  *
  * See rcu_read_lock() for more information.
  */
-#define rcu_read_unlock(x)     ({ ((void)(x)); preempt_enable(); })
+#define rcu_read_unlock(x)     ({ ((void)(x)); rcu_quiesce_enable(); })
 
 /*
  * So where is rcu_write_lock()?  It does not exist, as there is no
-- 
2.16.4


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