This is a re-submission of the patch I sent about a month ago.

While working on my code I realized that d_drop() might race against 
__d_lookup(). __d_drop() (which is called by d_drop() after acquiring 
the dcache_lock) is accessing dentry->d_flags to set the DCACHE_UNHASHED 
flag. This shouldn't be done without holding dentry->d_lock, like stated 
in dcache.h:

struct dentry {
	...
	unsigned int d_flags;		/* protected by d_lock */
         ...
};

Therefore d_drop() must acquire the dentry->d_lock. Likewise every use 
of __d_drop() must acquire that lock.

This patch fixes d_drop() and every grep'able __d_drop() use. This patch 
is against today's http://linux.bkbits.net/linux-2.5.

Comments please,
Jan