linux-kernel - Re: [PATCH v2 1/2] mbcache: decoupling the locking of local from global data

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Date:	Thu, 22 Aug 2013 15:33:32 +0000
From:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	T Makphaibulchoke <tmac@...com>, Theodore Ts'o <tytso@....edu>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>
Subject: Re: [PATCH v2 1/2] mbcache: decoupling the locking of local from
 global data

Thanks for the comments.

On 08/22/2013 04:53 PM, Linus Torvalds wrote:
> 
> Please don't do these ugly and pointless preprocessor macro expanders
> that hide what the actual operation is.
> 
> The DEBUG case seems to be just for your own testing anyway, so even
> that shouldn't exist in the merged version.
> 

Sorry, will clean them all up.

> 
> And this is really ugly. Again it's also then hidden behind the ugly macro.
> 
> First off, the thousand-time retry seems completely excessive. Does it
> actually need any retry AT ALL? If the hash entry changes, either you
> should retry forever, or if you feel that can result in livelocks
> (fair enough) and you need a fallback case to a bigger lock, then why
> not just do the fallback immediately?
> 
> More importantly, regardless of that retry issue, this seems to be
> abstracted at the wrong level, resulting in every single user of this
> repeating the same complex and hard-to-understand incantation:
> 

Looks like this is a misjudgement on my part.  There is really no need to guard against mb_cache_entry from moving to a different hash chain, as the shrinking and allocation function already protecting against each other thorugh mb_cache_spinlock.  The retry is not needed.

> 
> where the only difference is that the last one doesn't unlock
> afterwards because it runs in a loop with that LRU list lock held.
> Ugh.

Followed the above logic, all these pieces of code are also not necessary and could be just a simple unhash, as the original.

> 
> The locking logic also isn't explained anywhere, making the
> hard-to-read code even harder to read.


Will add comment, explaining the locking logic.

> 
>              Linus
> 

Thanks,
Mak.

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