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Re: [dm-devel] [PATCH 5/8] [dm-thin] Fix a race condition between discard bios and ordinary bios.



On Thu, Dec 13 2012 at  3:19pm -0500,
Joe Thornber <ejt redhat com> wrote:

> The deferred_set entries should not be incremented until the bio
> prison cells are held.  Otherwise quiescing a block for discard may
> end up waiting for a bio that's held in the discard bios cell.

This patch's subject and header needs help.  We've already fixed the
race with discards and normal bios in an earlier patch:
https://www.redhat.com/archives/dm-devel/2012-December/msg00010.html

This patch is purely about adapting dm-thin to use the new bio-prison
interface where the memory is now passed in rather than using a mempool
in bio-prison.  Two preallocated cells are now included in struct
thin_c; this allows the map function to not block performing allocations
(we want to avoid the cell allocation that is done in bio_detain).

The thin_c is allocated once in the constructor (thin_ctr).  Because the
thin_c is a shared resource, access to its cells must be serialized
using a new spinlock.

(NOTE: elevating the dm_bio_prison_cell structure from dm-bio-prison.c
to dm-bio-prison.h really should be part of this patch -- rather than
the previous patch).


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