• David Howells's avatar
    FS-Cache: Provide the ability to enable/disable cookies · 94d30ae9
    David Howells authored
    Provide the ability to enable and disable fscache cookies.  A disabled cookie
    will reject or ignore further requests to:
    	Acquire a child cookie
    	Invalidate and update backing objects
    	Check the consistency of a backing object
    	Allocate storage for backing page
    	Read backing pages
    	Write to backing pages
    but still allows:
    	Checks/waits on the completion of already in-progress objects
    	Uncaching of pages
    	Relinquishment of cookies
    Two new operations are provided:
     (1) Disable a cookie:
    	void fscache_disable_cookie(struct fscache_cookie *cookie,
    				    bool invalidate);
         If the cookie is not already disabled, this locks the cookie against other
         dis/enablement ops, marks the cookie as being disabled, discards or
         invalidates any backing objects and waits for cessation of activity on any
         associated object.
         This is a wrapper around a chunk split out of fscache_relinquish_cookie(),
         but it reinitialises the cookie such that it can be reenabled.
         All possible failures are handled internally.  The caller should consider
         calling fscache_uncache_all_inode_pages() afterwards to make sure all page
         markings are cleared up.
     (2) Enable a cookie:
    	void fscache_enable_cookie(struct fscache_cookie *cookie,
    				   bool (*can_enable)(void *data),
    				   void *data)
         If the cookie is not already enabled, this locks the cookie against other
         dis/enablement ops, invokes can_enable() and, if the cookie is not an
         index cookie, will begin the procedure of acquiring backing objects.
         The optional can_enable() function is passed the data argument and returns
         a ruling as to whether or not enablement should actually be permitted to
         All possible failures are handled internally.  The cookie will only be
         marked as enabled if provisional backing objects are allocated.
    A later patch will introduce these to NFS.  Cookie enablement during nfs_open()
    is then contingent on i_writecount <= 0.  can_enable() checks for a race
    between open(O_RDONLY) and open(O_WRONLY/O_RDWR).  This simplifies NFS's cookie
    handling and allows us to get rid of open(O_RDONLY) accidentally introducing
    caching to an inode that's open for writing already.
    One operation has its API modified:
     (3) Acquire a cookie.
    	struct fscache_cookie *fscache_acquire_cookie(
    		struct fscache_cookie *parent,
    		const struct fscache_cookie_def *def,
    		void *netfs_data,
    		bool enable);
         This now has an additional argument that indicates whether the requested
         cookie should be enabled by default.  It doesn't need the can_enable()
         function because the caller must prevent multiple calls for the same netfs
         object and it doesn't need to take the enablement lock because no one else
         can get at the cookie before this returns.
    Signed-off-by: David Howells <dhowells@redhat.com