linux-kernel - Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

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Date:   Thu, 20 Jul 2017 15:23:09 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jessica Yu <jeyu@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

On Thu, Jul 20, 2017 at 11:48:41AM -0400, Joe Lawrence wrote:
> On a related note, if we keep the allocations and memcpy, how about I
> shift around the attach/get calls like so:
> 
>   __klp_shadow_attach
>     set shadow variable member values
>     memcpy
>     add to hash
> 
>   klp_shadow_attach
>     alloc new shadow var
>     lock
>     call __klp_shadow_attach with new alloc
>     unlock
> 
>   klp_shadow_get_or_attach
>     be optimistic, call klp_shadow_get (if found, return it)
>     be pessimistic, alloc new shadow var
>     lock
>       call klp_shadow_get again
>       if unlikely found
>         kfree unneeded alloc
>       else
>         call __klp_shadow_attach with new alloc
>     unlock
>     return whichever shadow var we used
> 
> This way both calls can accept gfp_flags that may sleep, with the only
> downside that klp_shadow_get_or_attach may allocate an unnecessary
> shadow variable in the unlikely case that it's found on the second
> klp_shadow_get attempt (under the lock).  No more clunky "bool lock"
> flag either. :)

Sounds good to me!

-- 
Josh