[Linux-cachefs] [PATCH] fscache: fix race between enablement and dropping of object
NeilBrown
neilb at suse.com
Wed Nov 28 20:18:23 UTC 2018
On Wed, Nov 28 2018, David Howells wrote:
> NeilBrown <neilb at suse.com> wrote:
>
>> 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().
>
> Was there a backtrace for that?
Yes.
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
This is in SLES12-SP2 which is based on Linux-4.4
>
>> This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP
>
> Attempting to enable the cookie again would have that effect.
>
>> 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.
>
> That should work. The read op should just then fail with -ENOBUFS, causing
> the netfs to issue the read over the network instead.
>
> I think there's a more elegant solution to be had, but it'll take some thought
> as to how to achieve it.
Please let me know when you come up with something.
Thanks,
NeilBrown
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