linux/lib/percpu-rwsem.c
Oleg Nesterov 8ebe34731a percpu_rw_semaphore: add lockdep annotations
Add lockdep annotations.  Not only this can help to find the potential
problems, we do not want the false warnings if, say, the task takes two
different percpu_rw_semaphore's for reading.  IOW, at least ->rw_sem
should not use a single class.

This patch exposes this internal lock to lockdep so that it represents the
whole percpu_rw_semaphore.  This way we do not need to add another "fake"
->lockdep_map and lock_class_key.  More importantly, this also makes the
output from lockdep much more understandable if it finds the problem.

In short, with this patch from lockdep pov percpu_down_read() and
percpu_up_read() acquire/release ->rw_sem for reading, this matches the
actual semantics.  This abuses __up_read() but I hope this is fine and in
fact I'd like to have down_read_no_lockdep() as well,
percpu_down_read_recursive_readers() will need it.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Anton Arapov <anton@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-12-17 17:15:18 -08:00

165 lines
5 KiB
C

#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);
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 (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;
preempt_disable();
if (likely(!atomic_read(&brw->write_ctr))) {
__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.
*
* 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)
{
might_sleep();
if (likely(update_fast_ctr(brw, +1))) {
rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
return;
}
down_read(&brw->rw_sem);
atomic_inc(&brw->slow_read_ctr);
/* avoid up_read()->rwsem_release() */
__up_read(&brw->rw_sem);
}
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)))
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 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 */
atomic_inc(&brw->write_ctr);
/*
* 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().
*/
synchronize_sched_expedited();
/* exclude other writers, and block the new readers completely */
down_write(&brw->rw_sem);
/* 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 */
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();
/* the last writer unblocks update_fast_ctr() */
atomic_dec(&brw->write_ctr);
}