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Wed, 2 Oct 2019 08:19:31 +0000 (UTC) Subject: Re: [PATCH v1] s390x/tcg: MVCL: Exit to main loop if there are pending interrupts From: David Hildenbrand To: Richard Henderson , qemu-devel@nongnu.org References: <20191001181655.25948-1-david@redhat.com> <720221d3-84a6-9940-812e-b427acfc99ed@linaro.org> <42c78e3f-be24-a919-b1b9-0b52381a9214@redhat.com> <35424050-6341-4613-ff2f-536b4f35162b@redhat.com> Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+ xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl SE+xAvmumFBY Organization: Red Hat GmbH Message-ID: Date: Wed, 2 Oct 2019 10:19:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <35424050-6341-4613-ff2f-536b4f35162b@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.69]); Wed, 02 Oct 2019 08:19:32 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-s390x@nongnu.org, Cornelia Huck , Thomas Huth Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 02.10.19 09:04, David Hildenbrand wrote: > On 01.10.19 23:59, Richard Henderson wrote: >> On 10/1/19 12:47 PM, David Hildenbrand wrote: >>> On 01.10.19 21:17, Richard Henderson wrote: >>>> On 10/1/19 11:16 AM, David Hildenbrand wrote: >>>>> +static inline bool should_interrupt_instruction(CPUState *cs) >>>>> +{ >>>>> + /* >>>>> + * Something asked us to stop executing chained TBs, e.g., >>>>> + * cpu_interrupt() or cpu_exit(). >>>>> + */ >>>>> + if ((int32_t)atomic_read(&cpu_neg(cs)->icount_decr.u32) < 0) { >>>>> + return true; >>>>> + } >>>>> + >>>>> + /* We have a deliverable interrupt pending. */ >>>>> + if ((atomic_read(&cs->interrupt_request) & CPU_INTERRUPT_HARD)= && >>>>> + s390_cpu_has_int(S390_CPU(cs))) { >>>>> + return true; >>>>> + } >>>>> + return false; >>>>> +} >>>> >>>> The first condition should be true whenever the second condition is = true. >>> >>> @@ -1018,6 +1018,7 @@ static inline bool should_interrupt_instruction= (CPUState *cs) >>> /* We have a deliverable interrupt pending. */ >>> if ((atomic_read(&cs->interrupt_request) & CPU_INTERRUPT_HARD) &= & >>> s390_cpu_has_int(S390_CPU(cs))) { >>> + g_assert((int32_t)atomic_read(&cpu_neg(cs)->icount_decr.u32)= < 0); >>> return true; >>> } >>> return false; >>> >>> >>> ... >>> >>> >>> [ 60.109761] systemd[1]: Set hostname to . >>> ** >>> ERROR:/home/dhildenb/git/qemu/target/s390x/mem_helper.c:1021:should_i= nterrupt_instruction: assertion failed: ((int32_t)atomic_read(&cpu_neg(cs= )->icount_decr.u32) < 0) >>> >>> >>> A race? Roughly 20-30% pass the first but not the second check. And >>> in total, on a Fedora 30 boot, I can maybe see 30 calls of >>> should_interrupt_instruction() succeeding. >>> >>> I thought these could be pending interrupts that were not deliverable >>> when injected but are now deliverable. For these, >>> icount_decr.u32.high would already have been set to 0. >>> >>> OTOH, I guess we always exit the TB in case we change the "deliverabl= e" state >>> of an IRQ, e.g., after LPSW or LCTL. E.g., >>> >>> static DisasJumpType op_lctlg(DisasContext *s, DisasOps *o) >>> { >>> ... >>> /* Exit to main loop to reevaluate s390_cpu_exec_interrupt. */ >>> return DISAS_PC_STALE_NOCHAIN; >>> } >>> >>> Maybe really a race then - or we are not properly exiting back to the >>> main loop in all scenarios. >> >> I think that it's a race right here in should_interrupt_instruction. >> >> Notice, interrupt_request gets set before icount_decr. Indeed, the ba= rrier >> happens immediately before the set of icount_decr in cpu_exit(). >> >> (It is briefly confusing that we have a barrier in cpu_exit and not in >> tcg_handle_interrupt. But that's explained by the cpu_is_self -- no n= eed for a >> barrier for the current cpu. I also think we could usefully use >> atomic_store_release instead of a separate smp_wmb.) >> >> Therefore checking interrupt_request after checking icount_decr violat= es the >> ordering rules. >> >> This is confirmed, ish, by noticing putting a breakpoint at that secon= d return >> (or assert) and noticing that icount_decr.u16.hi =3D=3D -1. It did ge= t set by one >> of the other threads, and before gdb managed to stop the world. >=20 > I am still puzzled why I can trigger that many races. I'll do some more > investigation, however, if there would be a deliverable interrupt which > will not set icount_decr.u16.hi =3D=3D -1, then we would have to fix > something that ionstead. So the second check in this patch can indeed > go, thanks for clarifying! >=20 So there is one place I once introduced that is at least prone to a very rare race when CPUs are waking up: hw/intc/s390_flic.c:qemu_s390_flic_notify() We set "cs->interrupt_request |=3D CPU_INTERRUPT_HARD" for all CPUs - especially also STOPPED ones and trigger cpu_interrupt(cs, CPU_INTERRUPT_HARD) only if a) the CPU is running b) the CPU is sleeping and could eventually consume the interrupt There is a very small chance that a CPU would wake up and not get a cpu_interrupt(cs, CPU_INTERRUPT_HARD). In the worst case, the I/O interrupt would be delayed on that CPU, usually another CPU will pick it up - totally acceptable. If pending interrupts are deliverable is affected by the PSW mask and control registers: - lctl(g): We do a DISAS_PC_STALE_NOCHAIN - lpsw(e): We do a DISAS_NORETURN - stnosm: We do a DISAS_PC_STALE_NOCHAIN - ssm: We do a DISAS_PC_STALE_NOCHAIN Of course, the PSW will be changed when injecting exceptions, but that path should also properly check for other deliverable interrupts (I once added a deliver loop to s390_cpu_do_interrupt()). So I think the only option really is that I was seeing a bunch of the races you explained. --=20 Thanks, David / dhildenb