Re: [Xen-devel] [PATCH v2 1/2] x86/mem_sharing: reorder when pages are unlocked and released

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Apr 25, 2019 at 5:14 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> >>> On 15.04.19 at 19:51, <tamas@xxxxxxxxxxxxx> wrote:
> > Simply changing to get_page with dom_cow is not enough, I ran into
> > issues where the system still deadlocks due to the ordering of
> > put_page_and_type being called before mem_sharing_page_unlock. So the
> > ordering still has to be changed.
>
> Dead locks because of the ordering of releases (or locks or alike),
> not acquires? That looks suspicious to me, i.e. suggests yet
> another problem elsewhere.

FWIW page_lock has this warning:

 * We must never call _put_page_type() while holding a page_lock() for
 * that page; doing so may cause a deadlock under the right
 * conditions.

That said, I'm working on getting rid of using page_lock altogether
because it just over-complicates things for no good reason. Should be
able to send the new patches today.

Tamas

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