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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.04.19 at 15:33, <tamas@xxxxxxxxxxxxx> wrote:
> 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.

Oh, of course. I once again forgot that
mem_sharing_page_unlock() calls page_unlock().

> 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.

Oh, that's good news.

Jan



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