Re: [Patch 7/10]: ext3 online resize: SMP locking for group metadata
From: Pavel Machek (pavel_at_ucw.cz)
Date: 10/01/04
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Date: Fri, 1 Oct 2004 12:18:22 +0200 To: Stephen Tweedie <sct@redhat.com>
Hi!
> Fix the SMP locking for group metadata in online resize.
>
> Most of the resize is simply not dangerous from an SMP point of view, as
> we're adding new data beyond the end of the device and so we're
> guaranteed that no existing CPUs can already be using that data. But we
> need to be extremely careful about the ordering when enabling that new
> data.
>
> The key to this is sb->s_groups_count; when that is raised, it enables
> new space on a filesystem, and the kernel will suddenly start accessing
> all of the newly-created tables for that space.
>
> The precise rules we use are:
>
> * Writers of s_groups_count *must* hold lock_super
> AND
> * Writers must perform a smp_wmb() after updating all dependent
> data and before modifying the groups count
>
> * Readers must hold lock_super() over the access
> OR
> * Readers must perform an smp_rmb() after reading the groups count
> and before reading any dependent data.
>
> This leaves the hot path, where s_groups_count is referenced, requiring
> an smp_rmb(); but no other locking on that hot path is required.
Should not s_groups_count be atomic_t, then? Is it possible that
normal read sees only half-updated value?
[I know this is non-issue at least on i386, and probably on most
architectures as long as s_groups_count is aligned...
Pavel
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