linux-kernel - Re: [PATCH 1/5] locking: Introduce range reader/writer lock

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Message-Id: <20170403161917.4ef46f17@nimbus>
Date:   Mon, 3 Apr 2017 16:19:17 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     mingo@...nel.org, peterz@...radead.org, akpm@...ux-foundation.org,
        jack@...e.cz, kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/5] locking: Introduce range reader/writer lock

Le Tue, 28 Mar 2017 09:39:18 -0700,
Davidlohr Bueso <dave@...olabs.net> a écrit : 
> I'll wait to see if there are any more concerns and send a v2 with
> your corrections.

Hi Bavidlohr, I think there is a major issue regarding the task
catching a signal in wait_for_range().
I can see it when a thread is catching a signal, the process deadlock
in exit path.

Let's imagine all these tasks waiting for the complete range lock, so
range doesn't matter:

A get the lock in write
B want the read lock => B->blocking_range=1 (because of A)
C want the write lock => C->blocking_range=2 (A,B)
D want the read lock => D->blocking_range=3 (A,B,C)
=> C catch a signal and exit wait_for_ranges()
A release the lock 
        => B->blocking_range=0
        => D->blocking_range=2 (D has not seen C removal)
=> B get the lock
B release the lock  
        => D->blocking_range=1

D remains blocked while no one has the lock !

The issue is when removing a task from the interval tree, we
should decrement all the blocking_ranges of the task added to that
range after the one leaving... I can't see an easy fix for that :(

Am I right ?

Cheers,
Laurent.