linux-kernel - RE: [PATCH 2/3] ocfs2: Remove a useless spinlock

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Message-ID: <fe6c86050b3447758c09e1881582405f@AcuMS.aculab.com>
Date:   Tue, 19 Jul 2022 14:19:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe JAILLET' <christophe.jaillet@...adoo.fr>,
        Mark Fasheh <mark@...heh.com>,
        Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>
Subject: RE: [PATCH 2/3] ocfs2: Remove a useless spinlock

From: Christophe JAILLET
> Sent: 19 July 2022 14:25
> 
> Le 19/07/2022 à 12:24, David Laight a écrit :
> > From: Christophe JAILLET
> >> Sent: 19 July 2022 11:02
> >>
> >> 'node_map_lock' is a spinlock only used to protect calls to set_bit(),
> >> clear_bit() and test_bit().
> >>
> >> {set|clear}_bit() are already atomic and don't need this extra spinlock.
> >> test_bit() only reads the bitmap for a given bit.
> >>
> >> Remove this useless spinlock.
> >
> > It looks to me like the calling code is racy
> > unless there is another lock in the callers.
> 
> The call chains are:
>    ocfs2_recover_orphans()
>      ocfs2_mark_recovering_orphan_dir()
>        spin_lock(&osb->osb_lock);		<-- osb_lock spinlock
>        ocfs2_node_map_set_bit()			<-- uses node_map_lock
>        ...
>        spin_unlock(&osb->osb_lock);
>      ...
>      ocfs2_clear_recovering_orphan_dir()
>        ocfs2_node_map_clear_bit()		<-- uses node_map_lock
> 						    osb_lock is NOT taken
> 
> 
>    ocfs2_check_orphan_recovery_state()
>      spin_lock(&osb->osb_lock);			<-- osb_lock spinlock
>      ...
>      ocfs2_node_map_test_bit()			<-- uses node_map_lock
>      ...
>      spin_unlock(&osb->osb_lock);
> 
> 
> So the code looks already protected by the 'osb_lock' spinlock, but I
> don't know this code and ocfs2_mark_recovering_orphan_dir() looks tricky
> to me. (so some other eyes are much welcome)
> 
> > While map->map is protected, the result of test_bit()
> > is stale - so can't be used for much.
> >
> 
> Anyway, should there be a locking issue, it is there with or without my
> patch, right?

Indeed.

	David

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