blob: 652a8ee8efe95acaf1db28eeed40bba820787222 [file] [log] [blame]
#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/wait.h>
#include <linux/lockdep.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,
const char *name, struct lock_class_key *rwsem_key)
brw->fast_read_ctr = alloc_percpu(int);
if (unlikely(!brw->fast_read_ctr))
return -ENOMEM;
/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
__init_rwsem(&brw->rw_sem, name, rwsem_key);
atomic_set(&brw->write_ctr, 0);
atomic_set(&brw->slow_read_ctr, 0);
return 0;
void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
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 (atomic_read(write_ctr) == 0) 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, atomic_read(write_ctr) 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;
if (likely(!atomic_read(&brw->write_ctr))) {
__this_cpu_add(*brw->fast_read_ctr, val);
success = true;
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.
* Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
* percpu_up_read() does rwsem_release(). This pairs with the usage
* of ->rw_sem in percpu_down/up_write().
void percpu_down_read(struct percpu_rw_semaphore *brw)
if (likely(update_fast_ctr(brw, +1))) {
rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
/* avoid up_read()->rwsem_release() */
void percpu_up_read(struct percpu_rw_semaphore *brw)
rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
if (likely(update_fast_ctr(brw, -1)))
/* false-positive is possible but harmless */
if (atomic_dec_and_test(&brw->slow_read_ctr))
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 increments ->write_ctr to force the readers to switch to the
* slow mode, note the atomic_read() 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)
/* tell update_fast_ctr() there is a pending writer */
* 1. Ensures that write_ctr != 0 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().
/* exclude other writers, and block the new readers completely */
/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
/* 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)
/* release the lock, but the readers can't use the fast-path */
* Insert the barrier before the next fast-path in down_read,
* see W_R case in the comment above update_fast_ctr().
/* the last writer unblocks update_fast_ctr() */