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([2a0b:e7c0:0:107::49]) by smtp.gmail.com with ESMTPSA id b11sm15422489edw.76.2020.07.20.04.40.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jul 2020 04:40:21 -0700 (PDT) Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917 To: peterz@infradead.org, Oleg Nesterov Cc: Christian Brauner , christian@brauner.io, "Eric W. Biederman" , Linux kernel mailing list , Mel Gorman , Dave Jones , Paul Gortmaker References: <20200717122651.GA6067@redhat.com> <20200717124017.GB6067@redhat.com> <2c8ef23c-43b4-39d4-8e84-92769c948da9@kernel.org> <20200718171406.GB16791@redhat.com> <20200718174448.4btbjcvp6wbbdgts@wittgenstein> <20200720064326.GA6612@redhat.com> <20200720082657.GC6612@redhat.com> <20200720084106.GJ10769@hirez.programming.kicks-ass.net> <20200720105924.GE43129@hirez.programming.kicks-ass.net> <20200720112623.GF43129@hirez.programming.kicks-ass.net> From: Jiri Slaby Message-ID: <4df1e23a-8b07-8439-c54b-6ef9864aa78a@kernel.org> Date: Mon, 20 Jul 2020 13:40:20 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200720112623.GF43129@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=iso-8859-2 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20. 07. 20, 13:26, peterz@infradead.org wrote: > On Mon, Jul 20, 2020 at 12:59:24PM +0200, peterz@infradead.org wrote: >> On Mon, Jul 20, 2020 at 10:41:06AM +0200, Peter Zijlstra wrote: >>> On Mon, Jul 20, 2020 at 10:26:58AM +0200, Oleg Nesterov wrote: >>>> Peter, >>>> >>>> Let me add another note. TASK_TRACED/TASK_STOPPED was always protected by >>>> ->siglock. In particular, ttwu(__TASK_TRACED) must be always called with >>>> ->siglock held. That is why ptrace_freeze_traced() assumes it can safely >>>> do s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock). >>>> >>>> Can this change race with >>>> >>>> if (signal_pending_state(prev->state, prev)) { >>>> prev->state = TASK_RUNNING; >>>> } >>>> >>>> in __schedule() ? Hopefully not, signal-state is protected by siglock too. >>>> >>>> So I think this logic was correct even if it doesn't look nice. But "doesn't >>>> look nice" is true for the whole ptrace code ;) >>> >>> *groan*... another bit of obscure magic :-( >>> >>> let me go try and wake up and figure out how best to deal with this. > > This then? That seems to survive the strace thing. FWIW for me too. thanks, -- js