Messages in this thread | | | Date | Mon, 13 Oct 2014 03:14:59 +0100 | From | Al Viro <> | Subject | Re: WTF is d_add_ci() doing with negative dentries? |
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On Mon, Oct 13, 2014 at 12:56:11AM +0100, Anton Altaparmakov wrote:
> I am just wondering whether there might be error conditions in which we might end up with a (perhaps invalid) negative dentry in memory which could be found here? Probably not a problem especially now that d_invalidate() cannot fail any more.
Huh? Failing d_invalidate() on _negative_ dentry is flat-out impossible; it would be dropped just fine, and we wouldn't have found it in the first place. Check what it used to do all way back to 2.2.0: if (dentry->d_count) { if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode)) return -EBUSY; }
So unless you care about 2.1.something (2.0 didn't have dcache at all), this scenario isn't possible.
In any case, d_add_ci() users that might have negative dentries become positive cannot afford hashed negative dentries at all - at the very least they need to treat them as invalid in ->d_revalidate() in such situations. Exactly because having a hashed valid negative dentry for FuBaR after e.g. mkdir fubar will really hurt - mkdir won't have any way to know that old dentry was there; there was no variant of fubar in directory prior to that mkdir (FuBaR _was_ negative) and there's nothing to suggest looking for it. So it won't be noticed and it'll bloody well stay negative and hashed. I.e. stat FuBaR; mkdir fubar; stat FuBaR will have the second stat find dentry still hashed and valid negative.
You can get away with that if you store something like timestamp[1] of the parent directory in those negative dentries and check that in ->d_revalidate(). But that will work just fine, since d_add_ci() is serialized by ->i_mutex held on parent and whatever it was that added your "exact spelling" into directory has made all preexisting negative dentries invalid...
[1] for arbitrary values of time - e.g. on positive lookup set ->d_time to 0 on negative lookup set ->d_time to that of parent dentry on mkdir set ->d_time to 0 on unlink, rmdir and rename victim copy ->d_time from parent dentry on any directory modification bump its ->d_time on d_revalidate of negative dentry compare ->d_time with that of parent dentry and declare invalid on mismatch will do just fine.
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