linux-kernel - Re: [RFC PATCH 1/2] Add unlocked version of igrab.

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Date:	Sun, 27 Mar 2011 20:54:23 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Ryan Mallon <ryan@...ewatersys.com>
Cc:	viro@...iv.linux.org.uk, dchinner@...hat.com,
	Trond.Myklebust@...app.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] Add unlocked version of igrab.

On Mon, Mar 28, 2011 at 02:56:00PM +1300, Ryan Mallon wrote:
> Commit 250df6ed274d767da844a5d9f05720b804240197 "fs: protect
> inode->i_state with inode->i_lock" changes igrab to acquire inode->i_lock,
> however some callees, notably nfs_inode_add_request, already hold the lock
> when calling igrab.

I think a better solution to your problem is to notice that this is
called in the context of doing a write to an inode.  That means we
must already have a reference count on this inode, so it can't possibly
be in I_FREEING or I_WILL_FREE.  That means we can just call __iget()
instead ... except that __iget isn't exported to modules.

So we could just bump the refcount by hand ... or we could
EXPORT_SYMBOL(__iget).


Or Al's going to swear a lot about what NFS is doing here and how it's
utterly misdesigned.  Anyway, this patch could be part of the solution.


diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 85d7525..330cef3 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -390,7 +390,7 @@ static int nfs_inode_add_request(struct inode *inode, struct nfs_page *req)
 	error = radix_tree_insert(&nfsi->nfs_page_tree, req->wb_index, req);
 	BUG_ON(error);
 	if (!nfsi->npages) {
-		igrab(inode);
+		__iget(inode);
 		if (nfs_have_delegation(inode, FMODE_WRITE))
 			nfsi->change_attr++;
 	}

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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