From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9CA2A213E9C; Mon, 1 Jun 2026 10:52:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780311153; cv=none; b=n9NVoKYhUmi2BA9e84sQTesNTRLFfqse2hf1d/EyXgO08Xg6wtpg2OpPow+8//RCrXcX9JzRZhZFG66PGhUSY4whF53q5SFp8Nax/TQC3wQv1f9lh0OvxOVGH5MnKUI/xv7HAQz05FT7heKHWVoQxK/P54IMEq+SDZ5hk1gY3wo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780311153; c=relaxed/simple; bh=CtXY1b6tCj45p3S7bybTQ04xhyf53r4tD+62rDu/o+M=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=CYdAgcdF2x/Cv6dmQ21due92Hsd/qwjMPmmwPU6a2cLoq+ebmViRYfip4cYHrWU8/vOnzJi1IDu20R3jhMS4UXBMVxr7SMnRMUNTlCz/O8s//W+G7PzQGGh75rZKViMo+fmOLd38qvqMZWuEst7Th9lmvzz1FcZ9DDrNj+a6cT4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=vnuW0ArK; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="vnuW0ArK" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=z6ZbsAlUCa8+QBzbwfMgPm3rG9mHqJLXlJm0HxvP0B4=; b=vnuW0ArKiZNLzZwDwCYMO2LR/z iE/xF5GUJyxxnkgmNRhswn6LDOGAUWzYKEdAboD2RD+Kly9nCyQeYFiKI/m9x3RxgLWJUUayc/QkS zpa84krOgVeEJPtS7CsmZSo7ZktYe7tJTsVn3C3f10QoJzQMZ4X6CErbQpyVNK1RBH/z99HlrbLRo 4/Ezi81BQKDwZOfIH2YKR7kvEkyQFbLO06MNIVD9jrtRU3ImPhy4bbnlsqZtlmQR8aBdP42XUHwNd +dUCKtErNHMfL6ULXPnajMBPTnQWu7PQsC7PjS/ymomeIcVB8T8SUIthzPbC1rfc4c69+VWxR7DkO QdSYDRpQ==; Received: from [2001:8b0:10b:5:a9ac:3853:5e3b:fafd] (helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wU0Ft-0000000B6ka-2uPK; Mon, 01 Jun 2026 10:52:25 +0000 Message-ID: <14442dbf1e6d758fdd4756f2ed5c8cb2738582fb.camel@infradead.org> Subject: Re: [PATCH v2 01/20] locking/rt: Use raw_spin_lock_irqsave() in __rwbase_read_unlock() From: David Woodhouse To: Paolo Bonzini Cc: Sean Christopherson , Peter Zijlstra , Paul Durrant , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior , syzbot+208f7f3e5f59c11aeb90@syzkaller.appspotmail.com, Carsten Stollmaier Date: Mon, 01 Jun 2026 11:52:24 +0100 In-Reply-To: References: <20260529165114.748639-1-seanjc@google.com> <20260529165114.748639-2-seanjc@google.com> <20260529193214.GN3493090@noisy.programming.kicks-ass.net> <20260529193437.GB3568911@noisy.programming.kicks-ass.net> <20260529201335.GP3493090@noisy.programming.kicks-ass.net> <20260529203841.GC3568911@noisy.programming.kicks-ass.net> <1c3b224f-2780-4347-b72b-783586a9b0e1@redhat.com> <58d17a623c1c5c9c09bd91baa30510aaf42f899b.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-F2/tfdnQKjMoFnpMgqMn" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-F2/tfdnQKjMoFnpMgqMn Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sat, 2026-05-30 at 16:40 +0200, Paolo Bonzini wrote: > On Sat, May 30, 2026 at 3:04=E2=80=AFPM David Woodhouse wrote: > >=20 > > On Sat, 2026-05-30 at 12:26 +0200, Paolo Bonzini wrote: > > >=20 > > > Yeah, I think so. > > >=20 > > > The write side needs kvm->srcu so it would have to be yet another SRC= U. > > > I initially thought that sucks for the code that calls kvm_gpc_check(= ), > > > but maybe not because it simply replaces read_lock/read_unlock. > > >=20 > > > By using a seqcount for the data, SRCU only needs to be synchronized = in > > > gpc_unmap().=C2=A0 So, something like this: > >=20 > > It isn't just gpc_unmap() which does the invalidation. We also > > invalidate from the MMU notifier in gfn_to_pfn_cache_invalidate_start() > > which would also have to synchronize, wouldn't it? >=20 > You're right, the write_lock_irq() there drains the readers and that > is needed because khva is not pinned, only kmap()-ed. >=20 > That is already broken for the OOM case under PREEMPT_RT, where > rwlock_t becomes sleepable. But using SRCU would break it on > !PREEMPT_RT as well. I don't think 'sleepable' is the problem per se, is it? *Why* does the OOM killer use mmu_notifier_invalidate_range_start_nonblock()? Commit 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers") did say: There are several blockable mmu notifiers which might sleep in mmu_notifier_invalidate_range_start and that is a problem for the oom_reaper because it needs to guarantee a forward progress so it canno= t depend on any sleepable locks. But that was in 2018, when mmap_lock was an rw_semaphore. Is "sleepable" still a problem even when PREEMPT_RT where almost *everything* is now strictly sleepable? Wouldn't that mean drivers aren't even allowed to take their own spinlocks? I think the *real* constraint in the OOM path is that it mustn't block on anything which might be waiting for memory allocation. So waiting on an actual mutex is bad... but waiting for an rwlock which PREEMPT_RT just happens to have made sleepable... should be OK? And waiting for the in-guest CPU to respond to the IPI in Fred's patch should actually be OK too, but then so would returning -EAGAIN if any vCPUs really did need kicking. --=-F2/tfdnQKjMoFnpMgqMn Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 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