Re: [PATCH 2/2] tcg: use QTree instead of GTree

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Emilio Cota <cota@braap.org>
Cc: qemu-devel@nongnu.org, "Alex Bennée" <alex.bennee@linaro.org>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>
Subject: Re: [PATCH 2/2] tcg: use QTree instead of GTree
Date: Mon, 30 Jan 2023 09:27:10 +0000	[thread overview]
Message-ID: <Y9eNbnL+TgONAMPM@redhat.com> (raw)
In-Reply-To: <Y9b1UHyyPux7FKV5@cota-l14>

On Sun, Jan 29, 2023 at 05:38:08PM -0500, Emilio Cota wrote:
> On Wed, Jan 25, 2023 at 15:58:25 +0000, Daniel P. Berrangé wrote:
> > On Wed, Jan 11, 2023 at 12:34:29PM +0000, Daniel P. Berrangé wrote:
> > > On Tue, Jan 10, 2023 at 10:55:36PM -0500, Emilio Cota wrote:
> > > > qemu-user can hang in a multi-threaded fork. One common
> > > > reason is that when creating a TB, between fork and exec
> > > > we manipulate a GTree whose memory allocator (GSlice) is
> > > > not fork-safe.
> > > 
> > > BTW, I just checked latest glib status
> > > 
> > >   https://gitlab.gnome.org/GNOME/glib/-/issues/1079
> > > 
> > > it appears they're pretty close to deciding to delete the
> > > GSlice impl and always use system malloc.
> > 
> > They have now merged the code to delete the GSlice custom allocator.
> > 
> > So glib >= 2.76.0 should not exhibit a hang
> > 
> > > So if we do take this patch series it'll hopefully be a time
> > > limited thing to carry. 
> > 
> > So the question is whether the issue is critical enough that we want
> > to carry a workaround for a while, vs telling users to upgrade to
> > newer glib  (once 2.76 actually gets released)
> 
> That is great news!
> 
> Since this is a correctness issue, I think we should ship with qtree
> and use it when configuring with glib <2.76.0. For later glib versions
> we would just use gtree, e.g. via typedef + inline functions.
> 
> Once the minimum glib required by the configure script is >= 2.76.0,
> then we'd remove qtree.
> 
> If that sounds like a good plan, I can send a v2.

I'm fine with it, but be good to have an opinion here from the TCG
subsystem maintainers, CC'ing them


With regards,
Daniel
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