linux-ext4 - Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker)

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Message-ID: <Y8lGxkBrls6qQOdM@casper.infradead.org>
Date:   Thu, 19 Jan 2023 13:33:58 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
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Subject: Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker)

On Thu, Jan 19, 2023 at 03:23:08PM +0900, Byungchul Park wrote:
> Boqun wrote:
> > *	Looks like the DEPT dependency graph doesn't handle the
> > 	fair/unfair readers as lockdep current does. Which bring the
> > 	next question.
> 
> No. DEPT works better for unfair read. It works based on wait/event. So
> read_lock() is considered a potential wait waiting on write_unlock()
> while write_lock() is considered a potential wait waiting on either
> write_unlock() or read_unlock(). DEPT is working perfect for it.
> 
> For fair read (maybe you meant queued read lock), I think the case
> should be handled in the same way as normal lock. I might get it wrong.
> Please let me know if I miss something.

>From the lockdep/DEPT point of view, the question is whether:

	read_lock(A)
	read_lock(A)

can deadlock if a writer comes in between the two acquisitions and
sleeps waiting on A to be released.  A fair lock will block new
readers when a writer is waiting, while an unfair lock will allow
new readers even while a writer is waiting.