linux-kernel - Re: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed rwsems.

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Date:   Wed, 30 Mar 2022 13:23:23 +1100
From:   Imran Khan <imran.f.khan@...cle.com>
To:     Al Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>
Cc:     gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed
 rwsems.

Hello Al, Hello Tejun,

On 22/3/22 1:40 pm, Al Viro wrote:
> On Mon, Mar 21, 2022 at 09:20:06AM -1000, Tejun Heo wrote:
>> Hello,
>>
>> On Mon, Mar 21, 2022 at 05:55:53PM +0000, Al Viro wrote:
>>> Why bother with rwsem, when we don't need anything blocking under it?
>>> DEFINE_RWLOCK instead of DEFINE_SPINLOCK and don't make it static.
>>
>> Oh I mean, in case the common readers get way too hot, percpu_rwsem is a
>> relatively easy way to shift the burder from the readers to the writers. I
>> doubt we'll need that.
>>
>>> kernfs_walk_ns() - this is fucking insane; on the surface, it needs to
>>> be exclusive due to the use of the same static buffer.  It uses that
>>> buffer to generate a pathname, *THEN* walks over it with strsep().
>>> That's an... interesting approach, for the lack of other printable
>>> terms - we walk the chain of ancestors, concatenating their names
>>> into a buffer and separating those names with slashes, then we walk
>>> that buffer, searching for slashes...  WTF?
>>
>> It takes the @parent to walk string @path from. Where does it generate the
>> pathname?
> 
> Sorry, misread that thing - the reason it copies the damn thing at all is
> the use of strsep().  Yecch...  Rule of the thumb regarding strsep() use,
> be it in kernel or in the userland: don't.  It's almost never the right
> primitive to use.
> 
> Lookups should use qstr; it has both the length and place for hash.
> Switch kernfs_find_ns() to that (and lift the calculation of length
> into the callers that do not have it - note that kernfs_iop_lookup()
> does) and you don't need the strsep() shite (or copying) anymore.
> 
> That would allow for kernfs_walk_ns() to take kernfs_rename_lock shared.
> 
> HOWEVER, that's not the only lock needed there and this patchset is
> broken in that respect - it locks the starting node, then walks the
> path.  Complete with lookups in rbtrees of children in the descendents
> of that node and those are *not* locked.
>
Yes. This was wrong. I have tried to fix it by dropping the lock of
previous parent and taking the lock of current parent before each
invocation of kernfs_find_ns from kernfs_walk_ns. However this does not
look feasible because we are already under spinlock (kernfs_rename_lock).
This limitation will still be there even after changing
kernfs_rename_lock to a read-write lock.
I have thought of ways to fix this but have not yet got any solution.
I am checking further but in the mean time if you have some suggestions
please let me know.

Thanks
-- Imran