[RFC 04/16] padata: Detect deadlocks between main and helper threads

[Daniel Jordan <daniel.m.jordan@oracle.com>]

A caller of padata_do_multithreaded() can unwittingly introduce
deadlocks if it already holds lock(s) that thread_fn() takes.  Lockdep
can't detect such a dependency because it doesn't know that
padata_do_multithreaded() waits on the helper threads.

Use a lockdep_map to encode the dependency, following the pattern in
workqueue, CPU hotplug, and other parts of the kernel.  See
commit 4e6045f13478 ("workqueue: debug flushing deadlocks with lockdep")
for an example of a similar situation.

Each padata_do_multithreaded() callsite gets its own lock_class_key to
avoid false positives involving locks from different calls that don't
depend on each other.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
---
 include/linux/padata.h | 22 +++++++++++++++++++++-
 kernel/padata.c        | 15 +++++++++++++--
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/include/linux/padata.h b/include/linux/padata.h
index 2a9fa459463d..907d624a8ca4 100644
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -17,6 +17,7 @@
 #include <linux/spinlock.h>
 #include <linux/list.h>
 #include <linux/kobject.h>
+#include <linux/lockdep.h>
 
 #define PADATA_CPU_SERIAL   0x01
 #define PADATA_CPU_PARALLEL 0x02
@@ -188,6 +189,23 @@ extern void __init padata_init(void);
 static inline void __init padata_init(void) {}
 #endif
 
+#ifdef CONFIG_LOCKDEP
+
+#define padata_do_multithreaded(job)					      \
+({									      \
+	static struct lock_class_key __key;				      \
+	const char *__map_name = "padata master waiting";		      \
+									      \
+	padata_do_multithreaded_job((job), &__key, __map_name);		      \
+})
+
+#else
+
+#define padata_do_multithreaded(job)					      \
+	padata_do_multithreaded_job((job), NULL, NULL)
+
+#endif
+
 extern struct padata_instance *padata_alloc(const char *name);
 extern void padata_free(struct padata_instance *pinst);
 extern struct padata_shell *padata_alloc_shell(struct padata_instance *pinst);
@@ -195,7 +213,9 @@ extern void padata_free_shell(struct padata_shell *ps);
 extern int padata_do_parallel(struct padata_shell *ps,
 			      struct padata_priv *padata, int *cb_cpu);
 extern void padata_do_serial(struct padata_priv *padata);
-extern int padata_do_multithreaded(struct padata_mt_job *job);
+extern int padata_do_multithreaded_job(struct padata_mt_job *job,
+				       struct lock_class_key *key,
+				       const char *map_name);
 extern int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type,
 			      cpumask_var_t cpumask);
 #endif
diff --git a/kernel/padata.c b/kernel/padata.c
index d0876f861464..b458deb17121 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -64,6 +64,9 @@ struct padata_mt_job_state {
 	unsigned long		position;
 	unsigned long		remaining_size;
 	struct list_head	failed_works;
+#ifdef CONFIG_LOCKDEP
+	struct lockdep_map	lockdep_map;
+#endif
 };
 
 static void padata_free_pd(struct parallel_data *pd);
@@ -470,9 +473,11 @@ static void padata_mt_helper(struct work_struct *w)
 		ps->remaining_size -= size;
 
 		spin_unlock(&ps->lock);
+		lock_map_acquire(&ps->lockdep_map);
 
 		ret = job->thread_fn(position, end, job->fn_arg);
 
+		lock_map_release(&ps->lockdep_map);
 		spin_lock(&ps->lock);
 
 		if (ret) {
@@ -552,14 +557,16 @@ static void padata_undo(struct padata_mt_job_state *ps,
 }
 
 /**
- * padata_do_multithreaded - run a multithreaded job
+ * padata_do_multithreaded_job - run a multithreaded job
  * @job: Description of the job.
  *
  * See the definition of struct padata_mt_job for more details.
  *
  * Return: 0 or a client-specific nonzero error code.
  */
-int padata_do_multithreaded(struct padata_mt_job *job)
+int padata_do_multithreaded_job(struct padata_mt_job *job,
+				struct lock_class_key *key,
+				const char *map_name)
 {
 	/* In case threads finish at different times. */
 	static const unsigned long load_balance_factor = 4;
@@ -583,6 +590,7 @@ int padata_do_multithreaded(struct padata_mt_job *job)
 
 	spin_lock_init(&ps.lock);
 	init_completion(&ps.completion);
+	lockdep_init_map(&ps.lockdep_map, map_name, key, 0);
 	INIT_LIST_HEAD(&ps.failed_works);
 	ps.job		  = job;
 	ps.nworks	  = padata_work_alloc_mt(nworks, &ps, &works);
@@ -601,6 +609,9 @@ int padata_do_multithreaded(struct padata_mt_job *job)
 	ps.chunk_size = max(ps.chunk_size, job->min_chunk);
 	ps.chunk_size = roundup(ps.chunk_size, job->align);
 
+	lock_map_acquire(&ps.lockdep_map);
+	lock_map_release(&ps.lockdep_map);
+
 	list_for_each_entry(pw, &works, pw_list)
 		queue_work(system_unbound_wq, &pw->pw_work);
 
-- 
2.34.1