Please can you UNSUBSCRIBE me from this list?
thx
Le 13/11/2019 à 13:52, Al Viro a écrit :
> On Wed, Nov 13, 2019 at 09:01:36AM +0200, Amir Goldstein wrote:
>>> - if (d_really_is_negative(lower_dentry)) {
>>> + /*
>>> + * negative dentry can go positive under us here - its parent is not
>>> + * locked. That's OK and that could happen just as we return from
>>> + * ecryptfs_lookup() anyway. Just need to be careful and fetch
>>> + * ->d_inode only once - it's not stable here.
>>> + */
>>> + lower_inode = READ_ONCE(lower_dentry->d_inode);
>>> +
>>> + if (!lower_inode) {
>>> /* We want to add because we couldn't find in lower */
>>> d_add(dentry, NULL);
>>> return NULL;
>> Sigh!
>>
>> Open coding a human readable macro to solve a subtle lookup race.
>> That doesn't sound like a scalable solution.
>> I have a feeling this is not the last patch we will be seeing along
>> those lines.
>>
>> Seeing that developers already confused about when they should use
>> d_really_is_negative() over d_is_negative() [1] and we probably
>> don't want to add d_really_really_is_negative(), how about
>> applying that READ_ONCE into d_really_is_negative() and
>> re-purpose it as a macro to be used when races with lookup are
>> a concern?
> Would you care to explain what that "fix" would've achieved here,
> considering the fact that barriers are no-ops on UP and this is
> *NOT* an SMP race?
>
> And it's very much present on UP - we have
> fetch ->d_inode into local variable
> do blocking allocation
> check if ->d_inode is NULL now
> if it is not, use the value in local variable and expect it to be non-NULL
>
> That's not a case of missing barriers. At all. And no redefinition of
> d_really_is_negative() is going to help - it can't retroactively affect
> the value explicitly fetched into a local variable some time prior to
> that.
>
> There are other patches dealing with ->d_inode accesses, but they are
> generally not along the same lines. The problem is rarely the same...
>
