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