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[RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections

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  [RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections Boqun Feng <boqun.feng@gmail.com> - 2016-02-16 07:00 +0100
    [RFC v2 5/6] lockdep: LOCKED_ACCESS: Add proc interface for locked access class Boqun Feng <boqun.feng@gmail.com> - 2016-02-16 07:00 +0100

#1335040 — [RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections

Hi all,

This is the v2 for RCU_LOCKED_ACCESS.

Link of v1: http://article.gmane.org/gmane.linux.kernel/2143674

Changes since v1:

*	Define newly introduced irq_context helpers as macros when
	lockdep doesn't trace irq_context, because these helpers will be
	quite simple then and this could avoid some "defined but not
	used" warnings.

*	Introduce a new macro to static define a lock_class_key for RCU,
	in order to work around a compiler's "bug" about embracer-enclosed
	initializer list. (Paul Mckenney)


As a characteristic of RCU, read-side critical sections have a very
loose connection with rcu_dereference()s, which is you can only be sure
about an rcu_dereference() might be called in some read-side critical
section, but if code gets complex, you may not be sure which read-side
critical section exactly, this might be also an problem for some other
locking mechanisms, that is the critical sections protecting data and
the data accesses protected are not clearly correlated.

In this series, we are introducing LOCKED_ACCESS framework and based on
which, we implement the RCU_LOCKED_ACCESS functionality to give us a
clear hint: which rcu_dereference() happens in which RCU read-side
critical section. 

After this series applied, and if CONFIG_RCU_LOCKED_ACCESS=y, the proc
file /proc/locked_access/rcu will show all relationships collected so
far for rcu_read_lock() and their friends and rcu_dereference*().

Snippets of /proc/locked_access/rcu are as follow:

...(this rcu_dereference() happens after one rcu_read_lock())
...
ACQCHAIN 0xfdbf0c6aeea, 1 locks, irq_context 0:
  LOCK at [<ffffffff812b1115>] get_proc_task_net+0x5/0x140
    ACCESS TYPE 1 at kernel/pid.c:441
...
...(this rcu_dereference() happens after three rcu_read_lock())
...
ACQCHAIN 0xfe042af3bbfb2605, 3 locks, irq_context 0:
  LOCK at [<ffffffff81094b47>] SyS_kill+0x97/0x2a0
    LOCK at [<ffffffff8109286f>] kill_pid_info+0x1f/0x140
      LOCK at [<ffffffff81092605>] group_send_sig_info+0x5/0x130
        ACCESS TYPE 1 at kernel/signal.c:695
...


This patchset is based on v4.5-rc2 and consists of 6 patches(in which
patch 2-5 are the implementation of LOCKED_ACCESS):

1.	Introduce some functions of irq_context.

2.	Introduce locked access class and acqchain.

3.	Maintain the keys of acqchains.

4.	Introduce the entry point of LOCKED_ACCESS.

5.	Add proc interface for locked access class

6.	Enables LOCKED_ACCESS for RCU.

Tested by 0day and I also did a simple test on x86: build and boot a
kernel with RCU_LOCKED_ACCESS=y and CONFIG_PROVE_LOCKING=y and ran
several workloads(kernel building, git cloning, dbench), and
/proc/locked_access/rcu was able to collect the relationships between
~300 RCU read-critical sections and ~500 rcu_dereference*().

Looking forwards to any suggestion, comment and question ;-)

Regards,
Boqun

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#1335044 — [RFC v2 5/6] lockdep: LOCKED_ACCESS: Add proc interface for locked access class

Provide the proc filesystem interface for LOCKED_ACCESS, for a locked
access class whose name is <name>, there will be a file at
/proc/locked_access/<name> containing all the information the
LOCKED_ACCESS has collected so far.

Also add a macro to define a locked access class.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 kernel/locking/lockdep_internals.h |  17 +++++
 kernel/locking/lockdep_proc.c      | 127 +++++++++++++++++++++++++++++++++++++
 2 files changed, 144 insertions(+)

diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index 5e2e133..cacf869 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -241,4 +241,21 @@ struct locked_access_class {
 		.nr_acqchain_hlocks = 0,\
 		.nr_access_structs = 0, \
 	}
+
+extern int create_laclass_proc(const char *name,
+			   struct locked_access_class *laclass);
+
+#define DEFINE_CREATE_LACLASS_PROC(name) \
+static int __init ___create_##name##_laclass_proc(void) \
+{ \
+	return create_laclass_proc(#name, &name##_laclass); \
+} \
+late_initcall(___create_##name##_laclass_proc)
+
+/* Define a Locked Access Class and create its proc file */
+#define DEFINE_LACLASS(name) \
+	struct locked_access_class name##_laclass = \
+			INIT_LOCKED_ACCESS_DATA(name); \
+	EXPORT_SYMBOL(name##_laclass); \
+	DEFINE_CREATE_LACLASS_PROC(name)
 #endif /* CONFIG_LOCKED_ACCESS */
diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index dbb61a3..a278d1b 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -673,6 +673,130 @@ static const struct file_operations proc_lock_stat_operations = {
 };
 #endif /* CONFIG_LOCK_STAT */
 
+#ifdef CONFIG_LOCKED_ACCESS
+#include <linux/slab.h>
+static struct proc_dir_entry *locked_access_dir;
+static int seq_laclass_show(struct seq_file *m, void *v)
+{
+	struct locked_access_class *laclass;
+	loff_t *pos = v;
+	struct acqchain *acqchain;
+	struct locked_access_struct *s;
+	struct locked_access_location *loc;
+	unsigned long acq_ip;
+	int i;
+
+	laclass = (struct locked_access_class *)m->private;
+
+	/* Pair with the smp_store_release() in add_acqchain() */
+	if (*pos >= smp_load_acquire(&laclass->nr_acqchains) || *pos < 0)
+		return SEQ_SKIP;
+
+	acqchain = laclass->acqchains + *pos;
+
+	seq_printf(m, "ACQCHAIN 0x%llx, %d locks, irq_context %x:\n",
+			acqchain->chain_key, acqchain->depth,
+			acqchain->irq_context);
+	for (i = 0; i < acqchain->depth; i++) {
+		acq_ip = laclass->acqchain_hlocks[acqchain->base + i];
+		seq_printf(m, "%*sLOCK at [<%p>] %pSR\n",
+				(i+1) * 2, "", (void *) acq_ip,
+				(void *) acq_ip);
+	}
+
+	/* Pair with the list_add_tail_rcu() in add_locked_access() */
+	list_for_each_entry_lockless(s, &acqchain->accesses, list) {
+		loc = s->loc;
+		seq_printf(m, "%*sACCESS TYPE %x at %s:%ld\n",
+				(i+1) * 2, "", s->type, loc->filename,
+				loc->lineno);
+
+	}
+	return 0;
+}
+
+static void *seq_laclass_start(struct seq_file *m, loff_t *pos)
+{
+	struct locked_access_class *laclass;
+	loff_t *rpos;
+
+	laclass = (struct locked_access_class *)m->private;
+
+	/* Pair with the smp_store_release() in add_acqchain() */
+	if (*pos >= smp_load_acquire(&laclass->nr_acqchains) || *pos < 0)
+		return NULL;
+
+	rpos = kzalloc(sizeof(loff_t), GFP_KERNEL);
+
+	if (!rpos)
+		return NULL;
+
+	*rpos = *pos;
+
+	return rpos;
+}
+
+static void *seq_laclass_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct locked_access_class *laclass;
+	loff_t *rpos = v;
+
+	laclass = (struct locked_access_class *)m->private;
+
+	/* Pair with the smp_store_release() in add_acqchain() */
+	if (*pos + 1 < smp_load_acquire(&laclass->nr_acqchains) && *pos >= 0) {
+		*pos = ++*rpos;
+		return rpos;
+	}
+
+	return NULL;
+}
+
+static void seq_laclass_stop(struct seq_file *m, void *v)
+{
+	kfree(v);
+}
+
+static const struct seq_operations seq_laclass_ops = {
+	.start = seq_laclass_start,
+	.next = seq_laclass_next,
+	.stop = seq_laclass_stop,
+	.show = seq_laclass_show
+};
+
+static int proc_laclass_open(struct inode *inode, struct file *file)
+{
+	struct seq_file *seq;
+	int rc;
+
+	rc = seq_open(file, &seq_laclass_ops);
+
+	if (rc != 0)
+		return rc;
+
+	if (!PDE_DATA(inode))
+		return -ENOENT;
+
+	seq = file->private_data;
+	seq->private = PDE_DATA(inode);
+
+	return 0;
+}
+
+static const struct file_operations proc_laclass_operations = {
+	.open		= proc_laclass_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release
+};
+
+int create_laclass_proc(const char *name, struct locked_access_class *laclass)
+{
+	return !proc_create_data(name, S_IRUSR, locked_access_dir,
+			&proc_laclass_operations, laclass);
+}
+#endif /* CONFIG_LOCKED_ACCESS */
+
 static int __init lockdep_proc_init(void)
 {
 	proc_create("lockdep", S_IRUSR, NULL, &proc_lockdep_operations);
@@ -688,6 +812,9 @@ static int __init lockdep_proc_init(void)
 		    &proc_lock_stat_operations);
 #endif
 
+#ifdef CONFIG_LOCKED_ACCESS
+	locked_access_dir = proc_mkdir("locked_access", NULL);
+#endif /* CONFIG_LOCKED_ACCESS */
 	return 0;
 }
 
-- 
2.7.1

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