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Subject[PATCH 4/7] fscache: fix race between enablement and dropping of object
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From: NeilBrown <neilb@suse.com>

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 the wake in this patch will be handled correctly
by __fscache_read_or_alloc_page(), as it checks if ->backing_objects
is empty again, after waiting.

Customer which reported the hang, also report that the hang cannot be
reproduced with this fix.

The backtrace for the blocked process looked like:

PID: 29360 TASK: ffff881ff2ac0f80 CPU: 3 COMMAND: "zsh"
#0 [ffff881ff43efbf8] schedule at ffffffff815e56f1
#1 [ffff881ff43efc58] bit_wait at ffffffff815e64ed
#2 [ffff881ff43efc68] __wait_on_bit at ffffffff815e61b8
#3 [ffff881ff43efca0] out_of_line_wait_on_bit at ffffffff815e625e
#4 [ffff881ff43efd08] fscache_wait_for_deferred_lookup at ffffffffa04f2e8f [fscache]
#5 [ffff881ff43efd18] __fscache_read_or_alloc_page at ffffffffa04f2ffe [fscache]
#6 [ffff881ff43efd58] __nfs_readpage_from_fscache at ffffffffa0679668 [nfs]
#7 [ffff881ff43efd78] nfs_readpage at ffffffffa067092b [nfs]
#8 [ffff881ff43efda0] generic_file_read_iter at ffffffff81187a73
#9 [ffff881ff43efe50] nfs_file_read at ffffffffa066544b [nfs]
#10 [ffff881ff43efe70] __vfs_read at ffffffff811fc756
#11 [ffff881ff43efee8] vfs_read at ffffffff811fccfa
#12 [ffff881ff43eff18] sys_read at ffffffff811fda62
#13 [ffff881ff43eff50] entry_SYSCALL_64_fastpath at ffffffff815e986e

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/fscache/object.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 9edc920f651f..6d9cb1719de5 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -730,6 +730,9 @@ 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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