linux-kernel - [PATCH] fs: avoid unecessary smp_wmb() in dup

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Date:	Sat, 07 May 2011 08:30:01 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	Matthew Wilcox <matthew@....cx>, linux-fsdevel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Changli Gao <xiaosuo@...il.com>
Subject: [PATCH] fs: avoid unecessary smp_wmb() in dup_fd()

While copying old_fds array to new_fds, we dont need
rcu_assign_pointer() and its smp_wmb(), since we are the only thread
populating new_fds at this point.

A bit later, rcu_assign_pointer(newf->fdt, new_fdt) makes sure
our writes are committed to memory before new_fds content is visible to
other threads.

This removes following sparse warning :

fs/file.c:371:3: warning: incorrect type in assignment (different
address spaces)
fs/file.c:371:3:    expected struct file *<noident>
fs/file.c:371:3:    got struct file [noderef] <asn:4>*<noident>

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Cc: Matthew Wilcox <matthew@....cx>
Cc: linux-fsdevel@...r.kernel.org
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Changli Gao <xiaosuo@...il.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 fs/file.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index 4c6992d..8320a8e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -368,7 +368,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 			 */
 			FD_CLR(open_files - i, new_fdt->open_fds);
 		}
-		rcu_assign_pointer(*new_fds++, f);
+		*new_fds++ = f;
 	}
 	spin_unlock(&oldf->file_lock);
 


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