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locking/percpu-rwsem: Fix the comments outdated by rcu_sync 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>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Oleg Nesterov 2015-08-21 19:43:00 +02:00 committed by Paul E. McKenney
parent 001dac627f
commit f324a76324
1 changed files with 11 additions and 39 deletions
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50
kernel/locking/percpu-rwsem.c
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@ -39,27 +39,12 @@ 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)
{
@ -136,29 +121,15 @@ 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.
* 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.
*
* 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().
* The latter synchronises us with the preceding 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);
@ -178,8 +149,9 @@ 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);
}