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/* SPDX-License-Identifier: GPL-2.0 */
/*
* 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.
*
* 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_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)
#else
# define DEBUG_RWSEMS_WARN_ON(c)
#endif
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
* All writes to owner are protected by WRITE_ONCE() to make sure that
* store tearing can't happen as optimistic spinners may read and use
* the owner value concurrently without lock. Read from owner, however,
* may not need READ_ONCE() as long as the pointer value is only used
* for comparison and isn't being dereferenced.
*/
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
WRITE_ONCE(sem->owner, current);
}
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)
{
__rwsem_set_reader_owned(sem, current);
}
/*
* Return true if the a rwsem waiter can spin on the rwsem's owner
* and steal the lock, i.e. the lock is not anonymously owned.
* N.B. !owner is considered spinnable.
*/
static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
{
return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
}
/*
* Return true if rwsem is owned by an anonymous writer or readers.
*/
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)
{
}
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
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