linux-kernel - Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as potentially sleeping

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Date:   Wed, 19 Oct 2016 15:05:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     Christoph Hellwig <hch@....de>, akpm@...ux-foundation.org,
        joelaf@...gle.com, jszhang@...vell.com, joaodias@...gle.com,
        linux-mm@...ck.org, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as
        potentially sleeping

On Wed, Oct 19, 2016 at 12:15:41PM +0100, Chris Wilson wrote:
> On Tue, Oct 18, 2016 at 08:56:07AM +0200, Christoph Hellwig wrote:
> > This is how everyone seems to already use them, but let's make that
> > explicit.
> 
> Ah, found an exception, vmapped stacks:

Oh, fun.  So if we can't require vfree to be called from process context
we also can't use a mutex to wait for the vmap flushing.  Given that we
free stacks from the scheduler context switch I also fear there is no
good way to get a sleepable context there.

The only other idea I had was to use vmap_area_lock for the protection
that purge_lock currently provides, but that would require some serious
refactoring to avoid recursive locking first.