patch "/dev/mem: Add missing memory barriers for devmem_inode" added to char-misc-linus

This is a note to let you know that I've just added the patch titled

/dev/mem: Add missing memory barriers for devmem_inode

to my char-misc git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
in the char-misc-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From b34e7e298d7a5ed76b3aa327c240c29f1ef6dd22 Mon Sep 17 00:00:00 2001
From: Eric Biggers <[email protected]>
Date: Wed, 15 Jul 2020 23:05:53 -0700
Subject: /dev/mem: Add missing memory barriers for devmem_inode

WRITE_ONCE() isn't the correct way to publish a pointer to a data structure, since it doesn't include a write memory barrier. Therefore other tasks may see that the pointer has been set but not see that the pointed-to memory has finished being initialized yet. Instead a primitive with "release" semantics is needed.

Use smp_store_release() for this.

The use of READ_ONCE() on the read side is still potentially correct if there's no control dependency, i.e. if all memory being "published" is transitively reachable via the pointer itself. But this pairing is somewhat confusing and error-prone. So just upgrade the read side to smp_load_acquire() so that it clearly pairs with smp_store_release().

---
 drivers/char/mem.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

index 934c92dcb9ab..687d4af6945d 100644
@@ -814,7 +814,8 @@ static struct inode *devmem_inode;
 #ifdef CONFIG_IO_STRICT_DEVMEM
 void revoke_devmem(struct resource *res)
 {
-	struct inode *inode = READ_ONCE(devmem_inode);
+	/* pairs with smp_store_release() in devmem_init_inode() */
+	struct inode *inode = smp_load_acquire(&devmem_inode);
 
 	/*
 	 * Check that the initialization has completed. Losing the race

@@ -1028,8 +1029,11 @@ static int devmem_init_inode(void)
 		return rc;
 	}
 
-	/* publish /dev/mem initialized */
-	WRITE_ONCE(devmem_inode, inode);
+	/*
+	 * Publish /dev/mem initialized.
+	 * Pairs with smp_load_acquire() in revoke_devmem().
+	 */
+	smp_store_release(&devmem_inode, inode);
 
 	return 0;
 }

2.27.0