linux-kernel - Re: [RFC] inode.i_opflags - Usage of two different locking schemes

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Message-ID: <0f387f5b-a516-af45-856d-f38d1adfadf5@tu-dortmund.de>
Date:   Mon, 8 Mar 2021 15:05:33 +0100
From:   Alexander Lochmann <alexander.lochmann@...dortmund.de>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        Jan Kara <jack@...e.cz>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] inode.i_opflags - Usage of two different locking schemes



On 05.03.21 17:04, Theodore Ts'o wrote:
> On Fri, Mar 05, 2021 at 04:35:47PM +0100, Alexander Lochmann wrote:
>>
>>
>> On 05.03.21 16:18, Theodore Ts'o wrote:
>>> 1)  I don't see where i_opflags is being read in ipc/mqueue.c at all,
>>> either with or without i_rwsem.
>>>
>> It is read in fs/dcache.c
> 
> So why is this unique to the mqueue inode then?  It might be helpful
> to have explicit call stacks in the e-mail, in text form, when you
> resend to LKML.
It is unique to mqeue inode, because the control flow goes through 
ipc/mqueue.c where almost always the i_rwsem is taken.
Hence, we see more memory accesses to an mqueue inode with the i_rwsem.
The i_lock is less often hold compared to the i_rwsem.
We conclude the i_rwsem is needed. So it might not be a contradiction at 
all. It rather could be a flaw in our approach. :-/

Besides from our current discussion:
Does the i_lock protect i_opflags for both reading and writing?

Cheers,
Alex

> 
> That's because the HTML file is ***huge*** (1.7Meg), and I'm having
> trouble with my browser properly rendering it.  In any case, the html
> claims to be showing the counter examples and I'm still stuck on the
> *example*?
> 
> Cheers,
> 
> 					- Ted
> 

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