fs/dcache: Use release-acquire for name/length update (7088efa9) · Commits · 戴 / test

The code in __d_alloc() carefully orders filling in the NUL character of the name (and the length, hash, and the name itself) with assigning of the name itself. However, prepend_name() does not order the accesses to the ->name and ->len fields, other than on TSO systems. This commit therefore replaces prepend_name()'s READ_ONCE() of ->name with an smp_load_acquire(), which orders against the subsequent READ_ONCE() of ->len. Because READ_ONCE() now incorporates smp_read_barrier_depends(), prepend_name()'s smp_read_barrier_depends() is removed. Finally, to save a line, the smp_wmb()/store pair in __d_alloc() is replaced by smp_store_release(). Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: <linux-fsdevel@vger.kernel.org>

fs/dcache.c

+3 −7

Original line number	Diff line number	Diff line
		@@ -1636,8 +1636,7 @@ struct dentry __d_alloc(struct super_block sb, const struct qstr *name)
		dname[name->len] = 0;

		/* Make sure we always see the terminating NUL character */
		smp_wmb();
		dentry->d_name.name = dname;
		smp_store_release(&dentry->d_name.name, dname); /* ^^^ */

		dentry->d_lockref.count = 1;
		dentry->d_flags = 0;
		@@ -3047,17 +3046,14 @@ static int prepend(char *buffer, int buflen, const char *str, int namelen)
		* retry it again when a d_move() does happen. So any garbage in the buffer
		* due to mismatched pointer and length will be discarded.
		*
		* Data dependency barrier is needed to make sure that we see that terminating
		* NUL. Alpha strikes again, film at 11...
		* Load acquire is needed to make sure that we see that terminating NUL.
		*/
		static int prepend_name(char *buffer, int buflen, const struct qstr *name)
		{
		const char *dname = READ_ONCE(name->name);
		const char dname = smp_load_acquire(&name->name); / ^^^ */
		u32 dlen = READ_ONCE(name->len);
		char *p;

		smp_read_barrier_depends();

		*buflen -= dlen + 1;
		if (*buflen < 0)
		return -ENAMETOOLONG;

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