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-rw-r--r--include/linux/rwsem.h4
-rw-r--r--kernel/locking/rwsem-xadd.c2
-rw-r--r--kernel/locking/rwsem.c7
-rw-r--r--kernel/locking/rwsem.h95
4 files changed, 78 insertions, 30 deletions
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index ab93b6eae696..67dbb57508b1 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -45,10 +45,10 @@ struct rw_semaphore {
};
/*
- * Setting bit 0 of the owner field with other non-zero bits will indicate
+ * Setting bit 1 of the owner field but not bit 0 will indicate
* that the rwsem is writer-owned with an unknown owner.
*/
-#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-1L)
+#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-2L)
extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 01fcb807598c..09b180063ee1 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -180,7 +180,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* but it gives the spinners an early indication that the
* readers now have the lock.
*/
- rwsem_set_reader_owned(sem);
+ __rwsem_set_reader_owned(sem, waiter->task);
}
/*
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 776308d2fa9e..e586f0d03ad3 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -117,8 +117,9 @@ EXPORT_SYMBOL(down_write_trylock);
void up_read(struct rw_semaphore *sem)
{
rwsem_release(&sem->dep_map, 1, _RET_IP_);
- DEBUG_RWSEMS_WARN_ON(sem->owner != RWSEM_READER_OWNED);
+ DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED));
+ rwsem_clear_reader_owned(sem);
__up_read(sem);
}
@@ -181,7 +182,7 @@ void down_read_non_owner(struct rw_semaphore *sem)
might_sleep();
__down_read(sem);
- rwsem_set_reader_owned(sem);
+ __rwsem_set_reader_owned(sem, NULL);
}
EXPORT_SYMBOL(down_read_non_owner);
@@ -215,7 +216,7 @@ EXPORT_SYMBOL(down_write_killable_nested);
void up_read_non_owner(struct rw_semaphore *sem)
{
- DEBUG_RWSEMS_WARN_ON(sem->owner != RWSEM_READER_OWNED);
+ DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED));
__up_read(sem);
}
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index b9d0e72aa80f..bad2bca0268b 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,24 +1,30 @@
/* SPDX-License-Identifier: GPL-2.0 */
/*
- * The owner field of the rw_semaphore structure will be set to
- * RWSEM_READER_OWNED when a reader grabs the lock. A writer will clear
- * the owner field when it unlocks. A reader, on the other hand, will
- * not touch the owner field when it unlocks.
+ * The least significant 2 bits of the owner value has the following
+ * meanings when set.
+ * - RWSEM_READER_OWNED (bit 0): The rwsem is owned by readers
+ * - RWSEM_ANONYMOUSLY_OWNED (bit 1): The rwsem is anonymously owned,
+ * i.e. the owner(s) cannot be readily determined. It can be reader
+ * owned or the owning writer is indeterminate.
*
- * In essence, the owner field now has the following 4 states:
- * 1) 0
- * - lock is free or the owner hasn't set the field yet
- * 2) RWSEM_READER_OWNED
- * - lock is currently or previously owned by readers (lock is free
- * or not set by owner yet)
- * 3) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
- * - lock is owned by an anonymous writer, so spinning on the lock
- * owner should be disabled.
- * 4) Other non-zero value
- * - a writer owns the lock and other writers can spin on the lock owner.
+ * When a writer acquires a rwsem, it puts its task_struct pointer
+ * into the owner field. It is cleared after an unlock.
+ *
+ * When a reader acquires a rwsem, it will also puts its task_struct
+ * pointer into the owner field with both the RWSEM_READER_OWNED and
+ * RWSEM_ANONYMOUSLY_OWNED bits set. On unlock, the owner field will
+ * largely be left untouched. So for a free or reader-owned rwsem,
+ * the owner value may contain information about the last reader that
+ * acquires the rwsem. The anonymous bit is set because that particular
+ * reader may or may not still own the lock.
+ *
+ * That information may be helpful in debugging cases where the system
+ * seems to hang on a reader owned rwsem especially if only one reader
+ * is involved. Ideally we would like to track all the readers that own
+ * a rwsem, but the overhead is simply too big.
*/
-#define RWSEM_ANONYMOUSLY_OWNED (1UL << 0)
-#define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
+#define RWSEM_READER_OWNED (1UL << 0)
+#define RWSEM_ANONYMOUSLY_OWNED (1UL << 1)
#ifdef CONFIG_DEBUG_RWSEMS
# define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
@@ -44,15 +50,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
WRITE_ONCE(sem->owner, NULL);
}
+/*
+ * The task_struct pointer of the last owning reader will be left in
+ * the owner field.
+ *
+ * Note that the owner value just indicates the task has owned the rwsem
+ * previously, it may not be the real owner or one of the real owners
+ * anymore when that field is examined, so take it with a grain of salt.
+ */
+static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
+ struct task_struct *owner)
+{
+ unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED
+ | RWSEM_ANONYMOUSLY_OWNED;
+
+ WRITE_ONCE(sem->owner, (struct task_struct *)val);
+}
+
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
- /*
- * We check the owner value first to make sure that we will only
- * do a write to the rwsem cacheline when it is really necessary
- * to minimize cacheline contention.
- */
- if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
- WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+ __rwsem_set_reader_owned(sem, current);
}
/*
@@ -72,6 +89,25 @@ static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
{
return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
}
+
+#ifdef CONFIG_DEBUG_RWSEMS
+/*
+ * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
+ * is a task pointer in owner of a reader-owned rwsem, it will be the
+ * real owner or one of the real owners. The only exception is when the
+ * unlock is done by up_read_non_owner().
+ */
+#define rwsem_clear_reader_owned rwsem_clear_reader_owned
+static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
+{
+ unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
+ | RWSEM_ANONYMOUSLY_OWNED;
+ if (READ_ONCE(sem->owner) == (struct task_struct *)val)
+ cmpxchg_relaxed((unsigned long *)&sem->owner, val,
+ RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED);
+}
+#endif
+
#else
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
@@ -81,7 +117,18 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
}
+static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
+ struct task_struct *owner)
+{
+}
+
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
}
#endif
+
+#ifndef rwsem_clear_reader_owned
+static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
+{
+}
+#endif