[PATCH 1/7] rwsem-spinlock: Add killable versions of __down_read()

[Kirill Tkhai <ktkhai@virtuozzo.com>]

Rename __down_read() in __down_read_common() and teach it
to abort waiting in case of pending signals and killable
state argument passed.

Note, that we shouldn't wake anybody up in EINTR path, as:

We check for signal_pending_state() after (!waiter.task)
test and under spinlock. So, current task wasn't able to
be woken up. It may be in two cases: a writer is owner
of the sem, or a writer is a first waiter of the sem.

If a writer is owner of the sem, no one else may work
with it in parallel. It will wake somebody, when it
call up_write() or downgrade_write().

If a writer is the first waiter, it will be woken up,
when the last active reader releases the sem, and
sem->count became 0.

Also note, that set_current_state() may be moved down
to schedule() (after !waiter.task check), as all
assignments in this type of semaphore (including wake_up),
occur under spinlock, so we can't miss anything.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 include/linux/rwsem-spinlock.h  |    1 +
 kernel/locking/rwsem-spinlock.c |   37 ++++++++++++++++++++++++++++---------
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/include/linux/rwsem-spinlock.h b/include/linux/rwsem-spinlock.h
index ae0528b834cd..e784761a4443 100644
--- a/include/linux/rwsem-spinlock.h
+++ b/include/linux/rwsem-spinlock.h
@@ -32,6 +32,7 @@ struct rw_semaphore {
 #define RWSEM_UNLOCKED_VALUE		0x00000000
 
 extern void __down_read(struct rw_semaphore *sem);
+extern int __must_check __down_read_killable(struct rw_semaphore *sem);
 extern int __down_read_trylock(struct rw_semaphore *sem);
 extern void __down_write(struct rw_semaphore *sem);
 extern int __must_check __down_write_killable(struct rw_semaphore *sem);
diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
index 20819df98125..0848634c5512 100644
--- a/kernel/locking/rwsem-spinlock.c
+++ b/kernel/locking/rwsem-spinlock.c
@@ -126,7 +126,7 @@ __rwsem_wake_one_writer(struct rw_semaphore *sem)
 /*
  * get a read lock on the semaphore
  */
-void __sched __down_read(struct rw_semaphore *sem)
+int __sched __down_read_common(struct rw_semaphore *sem, int state)
 {
 	struct rwsem_waiter waiter;
 	unsigned long flags;
@@ -140,8 +140,6 @@ void __sched __down_read(struct rw_semaphore *sem)
 		goto out;
 	}
 
-	set_current_state(TASK_UNINTERRUPTIBLE);
-
 	/* set up my own style of waitqueue */
 	waiter.task = current;
 	waiter.type = RWSEM_WAITING_FOR_READ;
@@ -149,20 +147,41 @@ void __sched __down_read(struct rw_semaphore *sem)
 
 	list_add_tail(&waiter.list, &sem->wait_list);
 
-	/* we don't need to touch the semaphore struct anymore */
-	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
-
 	/* wait to be given the lock */
 	for (;;) {
 		if (!waiter.task)
 			break;
+		if (signal_pending_state(state, current))
+			goto out_nolock;
+		set_current_state(state);
+		raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 		schedule();
-		set_current_state(TASK_UNINTERRUPTIBLE);
+		raw_spin_lock_irqsave(&sem->wait_lock, flags);
 	}
 
-	__set_current_state(TASK_RUNNING);
+	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
  out:
-	;
+	return 0;
+
+out_nolock:
+	/*
+	 * We didn't take the lock, so that there is a writer, which
+	 * is owner or the first waiter of the sem. If it's a waiter,
+	 * it will be woken by current owner. Not need to wake anybody.
+	 */
+	list_del(&waiter.list);
+	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+	return -EINTR;
+}
+
+void __sched __down_read(struct rw_semaphore *sem)
+{
+	__down_read_common(sem, TASK_UNINTERRUPTIBLE);
+}
+
+int __sched __down_read_killable(struct rw_semaphore *sem)
+{
+	return __down_read_common(sem, TASK_KILLABLE);
 }
 
 /*