On 3/28/22 10:18, David Hildenbrand wrote:
On 26.03.22 14:40, Hillf Danton wrote:
In the slowpath of down for write, we bail out in case of signal received and
try to wake up any pending waiter but it makes no sense to wake up a write
waiter given any lock holder, either write or read.
But is handling this better really worth additional code and runtime
checks? IOW, does this happen often enough that we actually care about
optimizing this? I have no idea :)
The RFC is do nothing for wwaiter if any lock holder present - they will fill
their duty at lock release time.
Only for thoughts now.
Hillf
--- x/kernel/locking/rwsem.c
+++ y/kernel/locking/rwsem.c
@@ -418,6 +418,8 @@ static void rwsem_mark_wake(struct rw_se
waiter = rwsem_first_waiter(sem);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
+ if (RWSEM_LOCK_MASK & atomic_long_read(&sem->count))
+ return;
if (wake_type == RWSEM_WAKE_ANY) {
/*
* Mark writer at the front of the queue for wakeup.
--
That check isn't good enough. First of all, any reader count in
sem->count can be transient due to the fact that we do an unconditional
atomic_long_add() on down_read(). The reader may then remove its reader
count in the slow path. This patch may cause missed wakeup which is a
much bigger problem than spending a bit of cpu time to check for lock
availability and sleep again.
The write lock bit, however, is real. We do support the first writer in
the wait queue to spin on the lock when the handoff bit is set. So
waking up a writer when the rwsem is currently write-locked can still be
useful.
BTW, I didn't see this RFC patch in LKML. Is it only posted on linux-mm
originally?
Cheers,
Longman
