On Wed, Oct 16, 2024 at 04:53:45PM +0800, lizhe.67@xxxxxxxxxxxxx wrote: > On Wed, 16 Oct 2024 10:09:55 +0200, peterz@xxxxxxxxxxxxx wrote: > > > On Wed, Oct 16, 2024 at 12:35:58PM +0800, lizhe.67@xxxxxxxxxxxxx wrote: > > > From: Li Zhe <lizhe.67@xxxxxxxxxxxxx> > > > > > > In the current kernel rwsem implementation, there is an interface to > > > downgrade write lock to read lock, but there is no interface to upgrade > > > a read lock to write lock. This means that in order to acquire write > > > lock while holding read lock, we have to release the read lock first and > > > then acquire the write lock, which will introduce some troubles in > > > concurrent programming. This patch set provides the 'upgrade_read' interface > > > to solve this problem. This interface can change a read lock to a write > > > lock. > > > > upgrade-read is fundamentally prone to deadlocks. Imagine two concurrent > > invocations, each waiting for all readers to go away before proceeding > > to upgrade to a writer. > > > > Any solution to fixing that will end up being semantically similar to > > dropping the read lock and acquiring a write lock -- there will not be a > > single continuous critical section. > > According to the implementation of this patch, one of the invocation will Since the premise as described here is utter nonsense, I didn't get to actually reading the implementation -- why continue to waste time etc. > get '-EBUSY' in this case. If -EBUSY is obtained and the invocation thread > continues to retry instead of dropping the read lock and acquiring a write lock, > it may cause problems. Failure should drop the read lock, otherwise it is too easy to mess things up. > Of course, this patchset only try it's best to achieve a > single continuous critical section as much as possible, and there is no guarantee. As already stated, nothing like that was mentioned. > > As such, this interface makes no sense. > > This interface is just trying to reduce the overhead caused by the > additional checks, which is caused by non-continuous critical > sections, as much as possible. Rather than eliminating it in all > scenarios. So would it be better to change the error code to something > else? So that the caller will not retry this interface? You fail to quantify the gains. How am I supposed to know if the (significant?) increase in complexity is worth it? Why should I accept this increase in complexity for the sake of khugepaged, something which I care very little about?
