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Subject[PATCH 6/7] percpu-rwsem: fix the comments outdated by rcu_sync
Update the comments broken by the previous change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/locking/percpu-rwsem.c | 50 +++++++++--------------------------------
1 files changed, 11 insertions(+), 39 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 69a7314..705aefd 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -31,27 +31,12 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
}

/*
- * 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
+ * This is the fast-path for down_read/up_read. If it succeeds we rely
+ * on the barriers provided by rcu_sync_enter/exit; see the comments in
+ * percpu_down_write() and percpu_up_write().
*
* 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)
{
@@ -113,29 +98,15 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
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)
{
/*
- * 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().
+ * Make rcu_sync_is_idle() == F and thus disable the fast-path in
+ * percpu_down_read() and percpu_up_read(), and wait for gp pass.
*
- * 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().
+ * The latter synchronises us with the preceeding readers which used
+ * the fast-past, so we can not miss the result of __this_cpu_add()
+ * or anything else inside their criticial sections.
*/
rcu_sync_enter(&brw->rss);

@@ -154,8 +125,9 @@ 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().
+ * Enable the fast-path in percpu_down_read() and percpu_up_read()
+ * but only after another gp pass; this adds the necessary barrier
+ * to ensure the reader can't miss the changes done by us.
*/
rcu_sync_exit(&brw->rss);
}
--
1.5.5.1


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