linux-kernel - Re: [RFC 1/2] rwsem: introduce upgrade

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Message-ID: <ZxHs6WyGcJEfd_DI@infradead.org>
Date: Thu, 17 Oct 2024 22:06:49 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Waiman Long <llong@...hat.com>
Cc: Christoph Hellwig <hch@...radead.org>, lizhe.67@...edance.com,
	peterz@...radead.org, mingo@...hat.com, will@...nel.org,
	boqun.feng@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC 1/2] rwsem: introduce upgrade_read interface

On Thu, Oct 17, 2024 at 01:36:41PM -0400, Waiman Long wrote:
> > 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.
> 
> That means XFS only needs to upgrade to a write lock in certain cases only,
> not all of them.

Yes.  Basically when we detect a direct I/O writes needs to clear the
SUID bit or other metadata, or when it is an extending write.

> Right? In that case, read_try_upgrade() that attempts to
> upgrade to a write lock will be useful.

Yes.  But I also understand Peters reasoning that it will be very hard
to actually have a useful implementation that does better than just
unlocking (which is what we do currently).