linux-kernel - Re: [PATCH] fscache: fix race between enablement and dropping of object

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Date:   Thu, 29 Nov 2018 07:18:23 +1100
From:   NeilBrown <neilb@...e.com>
To:     David Howells <dhowells@...hat.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fscache: fix race between enablement and dropping of object

On Wed, Nov 28 2018, David Howells wrote:

> NeilBrown <neilb@...e.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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