Re: RFC: hold i_rwsem until aio completes

Waiman Long
2020-01-15T19:03:34Z

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