Re: [RFC 1/2] rwsem: introduce upgrade_read interface

[Christoph Hellwig]

On Wed, Oct 16, 2024 at 10:23:14AM -0400, Waiman Long wrote:
> Another alternative that I have been thinking about is a down_read() variant
> with intention to upgrade later. This will ensure that only one active
> reader is allowed to upgrade later. With this, upgrade_read() will always
> succeed, maybe with some sleeping, as long as the correct down_read() is
> used.

At least for the XFS use case where direct I/O takes a share lock
that needs to be replaced with an exclusive one for certain kinds of
I/O would be useless.  But then again we've survived without this
operation for a long time, despite the initial port bringing one over
from IRIX.