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Date: Fri, 30 Aug 2024 05:42:41 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
	Chandan Babu R <chandan.babu@oracle.com>,
	Christian Brauner <brauner@kernel.org>, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 01/10] iomap: handle a post-direct I/O invalidate race
 in iomap_write_delalloc_release
Message-ID: <20240830034241.GA25633@lst.de>
References: <20240827051028.1751933-1-hch@lst.de> <20240827051028.1751933-2-hch@lst.de> <20240827161416.GV865349@frogsfrogsfrogs> <20240828044848.GA31463@lst.de> <20240828161338.GH1977952@frogsfrogsfrogs> <20240829034626.GB3854@lst.de> <20240829142219.GC6216@frogsfrogsfrogs>
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On Thu, Aug 29, 2024 at 07:22:19AM -0700, Darrick J. Wong wrote:
> On Thu, Aug 29, 2024 at 05:46:26AM +0200, Christoph Hellwig wrote:
> > On Wed, Aug 28, 2024 at 09:13:38AM -0700, Darrick J. Wong wrote:
> > > Though we might have to revisit this for filesystems that don't take
> > > i_rwsem exclusively when writing -- is that a problem?  I guess if you
> > > had two threads both writing and punching the pagecache they could get
> > > into trouble, but that might be a case of "if it hurts don't do that".
> > 
> > No i_rwsem for buffered writes?  You can't really do that without hell
> > breaking lose.  At least not without another exclusive lock.
> 
> Well, i_rwsem in shared mode.  ISTR ext4 does inode_lock_shared and
> serializes on the folio lock, at least for non extending writes.

ext4 uses plain inode lock/unlock which is an exclusive i_rwsem.
Given that Posix requires the entire write to synchronized vs other
writes applications would break otherwise.