From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Brauner Subject: Re: [PATCH 1/2] pid: add pidfd_open() Date: Wed, 15 May 2019 17:30:42 +0200 Message-ID: <20190515153041.cshzaj7xhf2p4zv7@brauner.io> References: <20190515100400.3450-1-christian@brauner.io> <20190515143857.GB18892@redhat.com> <20190515144927.f2yxyi6w6lhn3xx7@brauner.io> <20190515151912.GE18892@redhat.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=brauner.io; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=aA/9xJj7LTMF1BWw8aRur3CJhiNYxnU3NsvAQ/tm1xk=; b=cVOJomieev/iQakiPOTryvyjScY68JwrcjZ9tnp9aeBqv3wRx4Edk6Zam5C4dpe1lU ogJ1bz/0uxayIY+dTGNRsGRKQE6a7udSOJYX/3GGdfJsVPiUlo9NIVE5wtEn+5hBwiSU gwP9J1DGbHYliH8LbuzUv/B66fmaNK+5+P4Nf0pkHXalxp2DHQe3TjDimH0gP6oUwq5l okb7EvO9DMTE9eTKyMdqlLNyrY47JfXoH49cTCN0H2L4KRmjYeMaDxi5FvoPvo3asXpV 5VjgjpZVUupeOdTmVGMkhRg4ZLi1nCgbsXBRTgk7m9FRnUbqV0616Vj7CPnf3pkdvUzD ZpUg== Content-Disposition: inline In-Reply-To: <20190515151912.GE18892@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Oleg Nesterov Cc: jannh@google.com, viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, arnd@arndb.de, dhowells@redhat.com, akpm@linux-foundation.org, cyphar@cyphar.com, ebiederm@xmission.com, elena.reshetova@intel.com, keescook@chromium.org, luto@amacapital.net, luto@kernel.org, tglx@linutronix.de, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org On Wed, May 15, 2019 at 05:19:13PM +0200, Oleg Nesterov wrote: > On 05/15, Christian Brauner wrote: > > > > On Wed, May 15, 2019 at 04:38:58PM +0200, Oleg Nesterov wrote: > > > > > > it seems that you can do a single check > > > > > > tsk = pid_task(p, PIDTYPE_TGID); > > > if (!tsk) > > > ret = -ESRCH; > > > > > > this even looks more correct if we race with exec changing the leader. > > > > The logic here being that you can only reach the thread_group leader > > from struct pid if PIDTYPE_PID == PIDTYPE_TGID for this struct pid? > > Not exactly... it is not that PIDTYPE_PID == PIDTYPE_TGID for this pid, > struct pid has no "type" or something like this. > > The logic is that pid->tasks[PIDTYPE_XXX] is the list of task which use > this pid as "XXX" type. > > For example, clone(CLONE_THREAD) creates a pid which has a single non- > empty list, pid->tasks[PIDTYPE_PID]. This pid can't be used as TGID or > SID. > > So if pid_task(PIDTYPE_TGID) returns non-NULL we know that this pid was > used for a group-leader, see copy_process() which does Ah, this was what I was asking myself when I worked on thread-specific signal sending. This clarifies quite a lot of things! Though I wonder how one reliably gets a the PGID or SID from a PIDTYPE_PID. > > if (thread_group_leader(p)) > attach_pid(p, PIDTYPE_TGID); > > > If we race with exec which changes the leader pid_task(TGID) can return > the old leader. We do not care, but this means that we should not check > thread_group_leader(). Nice! Thank you, Oleg! :) Christian