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author | NeilBrown <neilb@suse.com> | 2018-10-26 17:16:29 +1100 |
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committer | David Howells <dhowells@redhat.com> | 2018-11-30 15:57:31 +0000 |
commit | c5a94f434c82529afda290df3235e4d85873c5b4 (patch) | |
tree | ec6015d559022613143f67afcc00e3a190e47ca8 /fs/fscache/object.c | |
parent | 9a24ce5b66f9c8190d63b15f4473600db4935f1f (diff) | |
download | linux-c5a94f434c82529afda290df3235e4d85873c5b4.tar.gz linux-c5a94f434c82529afda290df3235e4d85873c5b4.tar.bz2 linux-c5a94f434c82529afda290df3235e4d85873c5b4.zip |
fscache: fix race between enablement and dropping of object
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>
Diffstat (limited to 'fs/fscache/object.c')
-rw-r--r-- | fs/fscache/object.c | 3 |
1 files changed, 3 insertions, 0 deletions
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. |