On Sat, 8 Jan 2022 08:46:17 -0800
> From: Tim Murray <timmur...@google.com>
>
> f2fs rw_semaphores work better if writers can starve readers,
> especially for the checkpoint thread, because writers are strictly
> more important than reader threads. This prevents significant priority
> inversion between low-priority readers that blocked while trying to
> acquire the read lock and a second acquisition of the write lock that
> might be blocking high priority work.
>
> Signed-off-by: Tim Murray <timmur...@google.com>
> Signed-off-by: Jaegeuk Kim <jaeg...@kernel.org>
> ---
...
> +/*
> + * An implementation of an rwsem that is explicitly unfair to readers. This
> + * prevents priority inversion when a low-priority reader acquires the read
> lock
> + * while sleeping on the write lock but the write lock is needed by
> + * higher-priority clients.
> + */
> +
> +struct f2fs_rwsem {
> + struct rw_semaphore internal_rwsem;
> + wait_queue_head_t read_waiters;
> +};
...
> +static inline void f2fs_down_read(struct f2fs_rwsem *sem)
> +{
> + wait_event(sem->read_waiters, down_read_trylock(&sem->internal_rwsem));
> +}
> +
> +static inline int f2fs_down_read_trylock(struct f2fs_rwsem *sem)
> +{
> + return down_read_trylock(&sem->internal_rwsem);
> +}
> +
> +static inline void f2fs_up_read(struct f2fs_rwsem *sem)
> +{
> + up_read(&sem->internal_rwsem);
> +}
> +
> +static inline void f2fs_down_write(struct f2fs_rwsem *sem)
> +{
> + down_write(&sem->internal_rwsem);
> +}
> +
> +static inline int f2fs_down_write_trylock(struct f2fs_rwsem *sem)
> +{
> + return down_write_trylock(&sem->internal_rwsem);
> +}
> +
> +static inline void f2fs_up_write(struct f2fs_rwsem *sem)
> +{
> + up_write(&sem->internal_rwsem);
> + wake_up_all(&sem->read_waiters);
> +}
> +
Here is my two cents, the unfair rwsem derived from lock_sock(), which has
no link to rwsem.
Only for thoughts now.
Hillf
struct unfair_rwsem {
spinlock_t lock;
int owner; /* < 0 writer, > 0 readers */
struct list_head reader, writer; /* read/write waiters */
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
};
struct unfair_rwsem_wait {
struct list_head node;
struct task_struct *task;
};
static void lock_unfair_rwsem(struct unfair_rwsem *us, int read)
{
struct unfair_rwsem_wait wait;
mutex_acquire(&us->dep_map, 0, 0, _RET_IP_);
might_sleep();
wait.task = current;
for (;;) {
spin_lock(&us->lock);
if (read) {
if (us->owner >= 0) {
us->owner++;
spin_unlock(&us->lock);
return;
}
list_add_tail(&wait.node, &us->reader);
} else {
if (us->owner == 0) {
us->owner--;
spin_unlock(&us->lock);
return;
}
list_add_tail(&wait.node, &us->writer);
}
set_current_state(TASK_UNINTERRUPTIBLE);
spin_unlock(&us->lock);
schedule();
}
}
void down_read_unfair_rwsem(struct unfair_rwsem *us)
{
lock_unfair_rwsem(us, 1);
}
void down_write_unfair_rwsem(struct unfair_rwsem *us)
{
lock_unfair_rwsem(us, 0);
}
static void unlock_unfair_rwsem(struct unfair_rwsem *us, int read)
{
struct list_head *head = NULL;
int all = 0;
spin_lock(&us->lock);
if (us->owner < 0) {
BUG_ON(read);
us->owner++;
BUG_ON(0 != us->owner);
if (!list_empty(&us->writer))
head = &us->writer;
else if (!list_empty(&us->reader)) {
head = &us->reader;
all = 1;
}
} else if (us->owner > 0) {
BUG_ON(!read);
BUG_ON(!list_empty(&us->reader));
us->owner--;
if (us->owner == 0)
if (!list_empty(&us->writer))
head = &us->writer;
} else
BUG_ON(1);
mutex_release(&us->dep_map, _RET_IP_);
if (head) {
struct unfair_rwsem_wait *wait;
do {
wait = list_first_entry(head, struct unfair_rwsem_wait,
node);
list_del(&wait->node);
wake_up_process(wait->task);
} while (all && !list_empty(head));
}
spin_unlock(&us->lock);
}
void up_write_unfair_rwsem(struct unfair_rwsem *us)
{
unlock_unfair_rwsem(us, 0);
}
void up_read_unfair_rwsem(struct unfair_rwsem *us)
{
unlock_unfair_rwsem(us, 1);
}
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