On 1/15/20 9:49 AM, Jason Gunthorpe wrote: > On Wed, Jan 15, 2020 at 03:33:47PM +0100, Peter Zijlstra wrote: >> On Wed, Jan 15, 2020 at 09:24:28AM -0400, Jason Gunthorpe wrote: >> >>> I was interested because you are talking about allowing the read/write side >>> of a rw sem to be held across a return to user space/etc, which is the >>> same basic problem. >> No it is not; allowing the lock to be held across userspace doesn't >> change the owner. This is a crucial difference, PI depends on there >> being a distinct owner. That said, allowing the lock to be held across >> userspace still breaks PI in that it completely wrecks the ability to >> analyze the critical section. > I'm not sure what you are contrasting? > > I was remarking that I see many places open code a rwsem using an > atomic and a completion specifically because they need to do the > things Christoph identified: > >> (1) no unlocking by another process than the one that acquired it >> (2) no return to userspace with locks held > As an example flow: obtain the read side lock, schedual a work queue, > return to user space, and unlock the read side from the work queue. We currently have down_read_non_owner() and up_read_non_owner() that perform the lock and unlock without lockdep tracking. Of course, that is a hack and their use must be carefully scrutinized to make sure that there is no deadlock or other potentially locking issues. Cheers, Longman
