Date: Thu, 28 May 2020 12:09:18 -0400 From: "dalias@...ifal.cx" <dalias@...ifal.cx> To: tangyizhou <tangyizhou@...wei.com> Cc: "musl@...ts.openwall.com" <musl@...ts.openwall.com>, "Wanghui (John)" <john.wanghui@...wei.com>, "Huangshuai (OSLab)" <elvis.huang@...wei.com> Subject: Re: Fix the return value of pthread_getschedparam in musl libc On Thu, May 28, 2020 at 02:27:55PM +0000, tangyizhou wrote: > > There's not such an issue. t->killlock is held so that this can't > > happen, and more importantly, so that the thread can't exit and > > the tid be reassigned to a new thread or process that would > > wrongly be acted upon. > > Sorry for late reply. > > t->killlock is held only in pthread functions, and it won't work in > the following situation. Assuming process A is running on CPU core > 0, process B is running on CPU core 1, process C is running on CPU > core 2. Process A calls pthread_getschedparam() to query the > information of process B. This is not possible. pthread_getschedparam operates on threads not processes. A pthread_t is only valid in the context of a process. There is simply no way to pass a pthread_t for a thread in a different process, because the identifiers are in a separate space. Two pthread_t values could be numerically identical but refer to completely different threads, or one of them be invalid, just because they're local to the process -- and mechanically, the address space -- they're in. > After SYS_sched_getparam succeeds and > before SYS_sched_getscheduler starts, we assume the scheduling > timeslice of A is running out, then A is put in the runqueue of the > kernel. This is a chance for C to call kill() to kill B. When A is > running again, SYS_sched_getparam returns -ESRCH. You seem to be confusing threads and processes. kill signals processes not threads. It's possible to send a signal to a particular thread; there's a standard interface to do this within a process, pthread_kill, and you could go outside the standard interfaces and do it cross-process using kernel tids with tkill. But that does not cause the thread to cease to exist. It makes a signal pending for the thread, and depending on the action for that signal, it may either cause a signal handler to run or cause *the whole process* to terminate. There is no way to forcibly terminate a single thread, from within the same process or a different one, short of UB or using trace/debugging type interfaces to attach to the process and do bad things to it. > Process B may be terminated due to other reasons when A is put in > the runqueue. For example, B is running and encounters a bus error, > then B is terminated because of SIGBUS signal. If SIGBUS is not caught, the whole *process* terminates, not the thread. > It very hard to see these situations, but they exist in a > theoretical way. There isn't such an issue for the implementation of > pthread_getschedparam() of glibc. These are non-issues based on your misunderstanding of what threads are. Rich
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