#include <linux/mutex.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/wait.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/errno.h>
int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
{
brw->fast_read_ctr = alloc_percpu(int);
if (unlikely(!brw->fast_read_ctr))
return -ENOMEM;
mutex_init(&brw->writer_mutex);
init_rwsem(&brw->rw_sem);
atomic_set(&brw->slow_read_ctr, 0);
init_waitqueue_head(&brw->write_waitq);
return 0;
}
void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
{
free_percpu(brw->fast_read_ctr);
brw->fast_read_ctr = NULL; /* catch use after free bugs */
}
/*
* This is the fast-path for down_read/up_read, it only needs to ensure
* there is no pending writer (!mutex_is_locked() check) and inc/dec the
* fast per-cpu counter. The writer uses synchronize_sched_expedited() to
* serialize with the preempt-disabled section below.
*
* The nontrivial part is that we should guarantee acquire/release semantics
* in case when
*
* R_W: down_write() comes after up_read(), the writer should see all
* changes done by the reader
* or
* W_R: down_read() comes after up_write(), the reader should see all
* changes done by the writer
*
* If this helper fails the callers rely on the normal rw_semaphore and
* atomic_dec_and_test(), so in this case we have the necessary barriers.
*
* But if it succeeds we do not have any barriers, mutex_is_locked() or
* __this_cpu_add() below can be reordered with any LOAD/STORE done by the
* reader inside the critical section. See the comments in down_write and
* up_write below.
*/
static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
{
bool success = false;
preempt_disable();
if (likely(!mutex_is_locked(&brw->writer_mutex))) {
__this_cpu_add(*brw->fast_read_ctr, val);
success = true;
}
preempt_enable();
return success;
}
/*
* Like the normal down_read() this is not recursive, the writer can
* come after the first percpu_down_read() and create the deadlock.
*/
void percpu_down_read(struct percpu_rw_semaphore *brw)
{
if (likely(update_fast_ctr(brw, +1)))
return;
down_read(&brw->rw_sem);
atomic_inc(&brw->slow_read_ctr);
up_read(&brw->rw_sem);
}
void percpu_up_read(struct percpu_rw_semaphore *brw)
{
if (likely(update_fast_ctr(brw, -1)))
return;
/* false-positive is possible but harmless */
if (atomic_dec_and_test(&brw->slow_read_ctr))
wake_up_all(&brw->write_waitq);
}
static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
{
unsigned int sum = 0;
int cpu;
for_each_possible_cpu(cpu) {
sum += per_cpu(*brw->fast_read_ctr, cpu);
per_cpu(*brw->fast_read_ctr, cpu) = 0;
}
return sum;
}
/*
* A writer takes ->writer_mutex to exclude other writers and to force the
* readers to switch to the slow mode, note the mutex_is_locked() check in
* update_fast_ctr().
*
* After that the readers can only inc/dec the slow ->slow_read_ctr counter,
* ->fast_read_ctr is stable. Once the writer moves its sum into the slow
* counter it represents the number of active readers.
*
* Finally the writer takes ->rw_sem for writing and blocks the new readers,
* then waits until the slow counter becomes zero.
*/
void percpu_down_write(struct percpu_rw_semaphore *brw)
{
/* also blocks update_fast_ctr() which checks mutex_is_locked() */
mutex_lock(&brw->writer_mutex);
/*
* 1. Ensures mutex_is_locked() is visible to any down_read/up_read
* so that update_fast_ctr() can't succeed.
*
* 2. Ensures we see the result of every previous this_cpu_add() in
* update_fast_ctr().
*
* 3. Ensures that if any reader has exited its critical section via
* fast-path, it executes a full memory barrier before we return.
* See R_W case in the comment above update_fast_ctr().
*/
synchronize_sched_expedited();
/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
/* block the new readers completely */
down_write(&brw->rw_sem);
/* wait for all readers to complete their percpu_up_read() */
wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
}
void percpu_up_write(struct percpu_rw_semaphore *brw)
{
/* allow the new readers, but only the slow-path */
up_write(&brw->rw_sem);
/*
* Insert the barrier before the next fast-path in down_read,
* see W_R case in the comment above update_fast_ctr().
*/
synchronize_sched_expedited();
mutex_unlock(&brw->writer_mutex);
}
