Hi Miklos,

I've checked that after applying the patch, the problem no longer occurs.

Since I'm running tests on RedHat 7.7, I had to slightly modify the 
patch, because on that kernel version locking around attr_version is 
done differently. I'm attaching modified patch, just in case.

Best regards,
Krzysztof Rusek

On 5/18/20 3:08 PM, Miklos Szeredi wrote:
> On Mon, May 4, 2020 at 1:00 PM Krzysztof Rusek <rusek@9livesdata.com> wrote:
>>
>> Hello,
>>
>> I'm working on a user-space file system implementation utilizing fuse
>> kernel module (and libfuse user-space library). This file system
>> implementation provides a custom ioctl operation that uses
>> fuse_lowlevel_notify_inval_inode() function (which translates to
>> fuse_notify_inval_inode() in kernel) to notify the kernel that the file
>> was changed by the ioctl. However, under certain circumstances,
>> fuse_notify_inval_inode() call is ineffective, resulting in incorrect
>> file attributes being cached by the kernel. The problem occurs when
>> ioctl() is executed in parallel with getattr().
>>
>> I noticed this problem on RedHat 7.7 (3.10.0-1062.el7.x86_64), but I
>> believe mainline kernel is affected as well.
>>
>> I think there is a bug in the way fuse_notify_inval_inode() invalidates
>> file attributes. If getattr() started before fuse_notify_inval_inode()
>> was called, then the attributes returned by user-space process may be
>> out of date, and should not be cached. But fuse_notify_inval_inode()
>> only clears attribute validity time, which does not prevent getattr()
>> result from being cached.
>>
>> More precisely, this bug occurs in the following scenario:
>>
>> 1. kernel starts handling ioctl
>> 2. file system process receives ioctl request
>> 3. kernel starts handling getattr
>> 4. file system process receives getattr request
>> 5. file system process executes getattr
>> 6. file system process executes ioctl, changing file state
>> 7. file system process invokes fuse_lowlevel_notify_inval_inode(), which
>> invalidates file attributes in kernel
>> 8. file system process sends ioctl reply, ioctl handling ends
>> 9. file system process sends getattr reply, attributes are incorrectly
>> cached in the kernel
>>
>> (Note that this scenario assumes that file system implementation is
>> multi-threaded, therefore allowing ioctl() and getattr() to be handled
>> in parallel.)
> 
> Can you please try the attached patch?
> 
> Thanks,
> Miklos
>