Re: [RFC PATCH] locking/rwsem: Add upgrade_read()
From: Waiman Long
Date: Wed Sep 22 2021 - 21:45:16 EST
On 9/22/21 9:16 PM, Boqun Feng wrote:
On Wed, Sep 22, 2021 at 03:36:57PM -0400, Waiman Long wrote:
Currently there are about 12 instances in the kernel where an up_read()
is immediately followed by a down_write() of the same lock. For example,
drivers/tty/n_tty.c: up_read(&tty->termios_rwsem);
drivers/tty/n_tty.c- down_write(&tty->termios_rwsem);
Since we have already provided a downgrade_write() function, we may as
well provide an upgrade_read() function to make the code easier to read
and the intention clearer.
If the current task is the only reader, the upgrade can be done by a
single atomic operation. If not, the upgrade will have to be done by a
separate up_read() call followed by a down_write(). In the former case,
the handoff bit is not considered and the waiter will have to wait a
bit longer to acquire the lock.
The new upgrade_read() function returns a value of 0 for safe upgrade
where rwsem protected data won't change. Otherwise a value of 1 is
returned to indicate unsafe upgrade where rwsem protected data may
change during the upgrade process.
For PREEMPT_RT, it falls back to up_read() followed by down_write()
for simplicity.
Some uses of down_write() with long lock hold time may be changed
to the following format in the future:
down_read()
/* check data */
if (upgrade_read()) {
/* unsafe upgrade, recheck data */
}
/* update data */
up_write();
As long as the "recheck data" and "update data" parts are relatively
short compared with the "check data" part, this conversion may help to
improve parallelism and reduce lock contention.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
include/linux/rwsem.h | 5 ++++
kernel/locking/rwsem.c | 53 ++++++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 352c6127cb90..8ece58224f25 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -207,6 +207,11 @@ extern void up_write(struct rw_semaphore *sem);
*/
extern void downgrade_write(struct rw_semaphore *sem);
+/*
+ * upgrade read lock to write lock
+ */
+extern int upgrade_read(struct rw_semaphore *sem);
+
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* nested locking. NOTE: rwsems are not allowed to recurse
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 000e8d5a2884..aeb5b0668304 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1203,6 +1203,29 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
return sem;
}
+/*
+ * Try to upgrade read lock to write lock
+ */
+static inline int __try_upgrade_read(struct rw_semaphore *sem)
+{
+ long count = atomic_long_read(&sem->count);
+
+ WARN_ON_ONCE(count & RWSEM_WRITER_LOCKED);
+
+ /*
+ * When upgrading from shared to exclusive ownership,
+ * anything inside the write-locked region cannot leak
+ * into the read side. Use an ACQUIRE semantics.
+ */
+ if (((count & RWSEM_READER_MASK) == RWSEM_READER_BIAS) &&
+ atomic_long_try_cmpxchg_acquire(&sem->count, &count,
+ count - RWSEM_READER_BIAS + RWSEM_WRITER_LOCKED)) {
+ rwsem_set_owner(sem);
+ return 1;
+ }
+ return 0;
+}
+
/*
* lock for reading
*/
@@ -1438,6 +1461,11 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
rwbase_write_downgrade(&sem->rwbase);
}
+static inline int __try_upgrade_read(struct rw_semaphore *sem)
+{
+ return 0;
+}
+
/* Debug stubs for the common API */
#define DEBUG_RWSEMS_WARN_ON(c, sem)
@@ -1581,6 +1609,31 @@ void downgrade_write(struct rw_semaphore *sem)
}
EXPORT_SYMBOL(downgrade_write);
+/*
+ * Upgrade read lock to write lock
+ *
+ * Return: 0 when upgrade is safe, i.e. rwsem protected data do not change;
+ * 1 when upgrade is unsafe as rwsem protected data may have changed.
+ */
+int upgrade_read(struct rw_semaphore *sem)
+{
+ if (__try_upgrade_read(sem)) {
+ rwsem_release(&sem->dep_map, _RET_IP_);
+ rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
+ return 0;
+ }
+
+ /*
+ * We cannot directly upgrade to the write lock, just do a regular
+ * up_read() and down_write() sequence. The data protected by the
+ * rwsem may have changed before the write lock is acquired.
+ */
+ down_read(sem);
+ up_write(sem);
Confused, the comment says up_read()+down_write(), however the code is
down_read()+up_write().
Thanks for catching that typo. My bad.
Besides, I don't like the idea that the value may have changed before
the write lock is acquired if we call it "upgrade". Maybe we want api
like down_read_upgradable(), which can be held in parallel with other
down_read() but no other down_read_upgradable(), and one can only
upgrade the read-side critical section created by
down_read_upgradable(). For implementation, that means we need to have
one extra bit for upgradable. Thoughts?
I like your idea. There are spare bits available and we can dedicate one
bit for that purpose. After successfully acquire the bit the reader can
probably spin a little bit and then insert itself to the head of the
wait queue to sleep. The last exiting reader can wake it up to acquire
the write lock.
I will probably use "try_upgrade_read() to indicate that the attempt may
fail.
Thanks for the suggestion.
Cheers,
Longman