fscache: Fix invalidation/lookup race

If an NFS file is opened for writing and closed, fscache_invalidate() will
be asked to invalidate the file - however, if the cookie is in the
LOOKING_UP state (or the CREATING state), then request to invalidate
doesn't get recorded for fscache_cookie_state_machine() to do something
with.

Fix this by making __fscache_invalidate() set a flag if it sees the cookie
is in the LOOKING_UP state to indicate that we need to go to invalidation.
Note that this requires a count on the n_accesses counter for the state
machine, which that will release when it's done.

fscache_cookie_state_machine() then shifts to the INVALIDATING state if it
sees the flag.

Without this, an nfs file can get corrupted if it gets modified locally and
then read locally as the cache contents may not get updated.

Fixes: d24af13e2e23 ("fscache: Implement cookie invalidation")
Reported-by: Max Kellermann <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Tested-by: Max Kellermann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]-ag [1]

fs/fscache/cookie.c

···
 	}
 
 	fscache_see_cookie(cookie, fscache_cookie_see_active);
-	fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_ACTIVE);
+	spin_lock(&cookie->lock);
+	if (test_and_clear_bit(FSCACHE_COOKIE_DO_INVALIDATE, &cookie->flags))
+		__fscache_set_cookie_state(cookie,
+					   FSCACHE_COOKIE_STATE_INVALIDATING);
+	else
+		__fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_ACTIVE);
+	spin_unlock(&cookie->lock);
+	wake_up_cookie_state(cookie);
 	trace = fscache_access_lookup_cookie_end;
 
 out:
···
 			cookie->volume->cache->ops->withdraw_cookie(cookie);
 			spin_lock(&cookie->lock);
 		}
+
+		if (test_and_clear_bit(FSCACHE_COOKIE_DO_INVALIDATE, &cookie->flags))
+			fscache_end_cookie_access(cookie, fscache_access_invalidate_cookie_end);
 
 		switch (state) {
 		case FSCACHE_COOKIE_STATE_RELINQUISHING:
···
 		return;
 
 	case FSCACHE_COOKIE_STATE_LOOKING_UP:
+		__fscache_begin_cookie_access(cookie, fscache_access_invalidate_cookie);
+		set_bit(FSCACHE_COOKIE_DO_INVALIDATE, &cookie->flags);
+		fallthrough;
 	case FSCACHE_COOKIE_STATE_CREATING:
 		spin_unlock(&cookie->lock);
 		_leave(" [look %x]", cookie->inval_counter);

include/linux/fscache.h

···
 #define FSCACHE_COOKIE_DO_PREP_TO_WRITE	12		/* T if cookie needs write preparation */
 #define FSCACHE_COOKIE_HAVE_DATA	13		/* T if this cookie has data stored */
 #define FSCACHE_COOKIE_IS_HASHED	14		/* T if this cookie is hashed */
+#define FSCACHE_COOKIE_DO_INVALIDATE	15		/* T if cookie needs invalidation */
 
 	enum fscache_cookie_state	state;
 	u8				advice;		/* FSCACHE_ADV_* */