Commit 38026d1a authored by NeilBrown's avatar NeilBrown Committed by Greg Kroah-Hartman
Browse files

fscache: fix race between enablement and dropping of object

[ Upstream commit c5a94f43 ]

It was observed that a process blocked indefintely in
__fscache_read_or_alloc_page(), waiting for FSCACHE_COOKIE_LOOKING_UP
to be cleared via fscache_wait_for_deferred_lookup().

At this time, ->backing_objects was empty, which would normaly prevent
__fscache_read_or_alloc_page() from getting to the point of waiting.
This implies that ->backing_objects was cleared *after*
__fscache_read_or_alloc_page was was entered.

When an object is "killed" and then "dropped",
FSCACHE_COOKIE_LOOKING_UP is cleared in fscache_lookup_failure(), then
KILL_OBJECT and DROP_OBJECT are "called" and only in DROP_OBJECT is
->backing_objects cleared.  This leaves a window where
something else can set FSCACHE_COOKIE_LOOKING_UP and
__fscache_read_or_alloc_page() can start waiting, before
->backing_objects is cleared

There is some uncertainty in this analysis, but it seems to be fit the
observations.  Adding th...
parent b718d6be
......@@ -715,6 +715,9 @@ static const struct fscache_state *fscache_drop_object(struct fscache_object *ob
if (awaken)
wake_up_bit(&cookie->flags, FSCACHE_COOKIE_INVALIDATING);
if (test_and_clear_bit(FSCACHE_COOKIE_LOOKING_UP, &cookie->flags))
wake_up_bit(&cookie->flags, FSCACHE_COOKIE_LOOKING_UP);
/* Prevent a race with our last child, which has to signal EV_CLEARED
* before dropping our spinlock.
......
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