linux-kernel - Re: [RFC PATCH] binder: Don't require the binder lock when killed in binder_thread

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Date:   Fri, 31 Mar 2017 21:29:44 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     arve@...roid.com, riandrews@...roid.com, tkjos@...gle.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] binder: Don't require the binder lock when killed in
 binder_thread_read()

On Fri, Mar 31, 2017 at 10:53:41AM -0700, Douglas Anderson wrote:
> Sometimes when we're out of memory the OOM killer decides to kill a
> process that's in binder_thread_read().  If we happen to be waiting
> for work we'll get the kill signal and wake up.  That's good.  ...but
> then we try to grab the binder lock before we return.  That's bad.
> 
> The problem is that someone else might be holding the one true global
> binder lock.  If that one other process is blocked then we can't
> finish exiting.  In the worst case, the other process might be blocked
> waiting for memory.  In that case we'll have a really hard time
> exiting.
> 
> On older kernels that don't have the OOM reaper (or something
> similar), like kernel 4.4, this is a really big problem and we end up
> with a simple deadlock because:
> * Once we pick a process to OOM kill we won't pick another--we first
>   wait for the process we picked to die.  The reasoning is that we've
>   given the doomed process access to special memory pools so it can
>   quit quickly and we don't have special pool memory to go around.
> * We don't have any type of "special access donation" that would give
>   the mutex holder our special access.
> 
> On kernel 4.4 w/ binder patches, we easily see this happen:

<snip>

How does your change interact with the recent "break up the binder big
lock" patchset:
	https://android-review.googlesource.com/#/c/354698/

Have you tried that series out to see if it helps out any?

thanks,

greg k-h