On Tue, Apr 16, 2024 at 05:50:14PM +0200, Peter Zijlstra wrote:
On Tue, Apr 16, 2024 at 10:14:21AM +0200, Peter Zijlstra wrote:
Some aspects of the implementation may deserve particular comment:
In the interest of performance, each object is governed only by a single spinlock. However, NTSYNC_IOC_WAIT_ALL requires that the state of multiple objects be changed as a single atomic operation. In order to achieve this, we first take a device-wide lock ("wait_all_lock") any time we are going to lock more than one object at a time.
The maximum number of objects that can be used in a vectored wait, and therefore the maximum that can be locked simultaneously, is 64. This number is NT's own limit.
AFAICT:
spin_lock(&dev->wait_all_lock); list_for_each_entry(entry, &obj->all_waiters, node) for (i=0; i<count; i++) spin_lock_nest_lock(q->entries[i].obj->lock, &dev->wait_all_lock);
Where @count <= NTSYNC_MAX_WAIT_COUNT.
So while this nests at most 65 spinlocks, there is no actual bound on the amount of nested lock sections in total. That is, all_waiters list can be grown without limits.
Can we pretty please make wait_all_lock a mutex ?
Hurmph, it's worse, you do that list walk while holding some obj->lock spinlokc too. Still need to figure out how all that works....