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Date:   Sun, 17 May 2020 17:18:53 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        linux-ia64@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [PATCH v2] ia64: enable HAVE_COPY_THREAD_TLS, switch to
 kernel_clone_args

On Sun, May 17, 2020 at 05:16:35PM +0200, Christian Brauner wrote:
> This is part of a larger series that aims at getting rid of the
> copy_thread()/copy_thread_tls() split that makes the process creation
> codepaths in the kernel more convoluted and error-prone than they need
> to be.
> I'm converting all the remaining arches that haven't yet switched and
> am collecting individual acks. Once I have them, I'll send the whole series
> removing the copy_thread()/copy_thread_tls() split, the
> HAVE_COPY_THREAD_TLS define and the legacy do_fork() helper. The only
> kernel-wide process creation entry point for anything not going directly
> through the syscall path will then be based on struct kernel_clone_args.
> No more danger of weird process creation abi quirks between architectures
> hopefully, and easier to maintain overall.
> It also unblocks implementing clone3() on architectures not support
> copy_thread_tls(). Any architecture that wants to implement clone3()
> will need to select HAVE_COPY_THREAD_TLS and thus need to implement
> copy_thread_tls(). So both goals are connected but independently
> beneficial.
> 
> HAVE_COPY_THREAD_TLS means that a given architecture supports
> CLONE_SETTLS and not setting it should usually mean that the
> architectures doesn't implement it but that's not how things are. In
> fact all architectures support CLONE_TLS it's just that they don't
> follow the calling convention that HAVE_COPY_THREAD_TLS implies. That
> means all architectures can be switched over to select
> HAVE_COPY_THREAD_TLS. Once that is done we can remove that macro (yay,
> less code), the unnecessary do_fork() export in kernel/fork.c, and also
> rename copy_thread_tls() back to copy_thread(). At this point
> copy_thread() becomes the main architecture specific part of process
> creation but it will be the same layout and calling convention for all
> architectures. (Once that is done we can probably cleanup each
> copy_thread() function even more but that's for the future.)
> 
> Since ia64 does support CLONE_SETTLS there's no reason to not select
> HAVE_COPY_THREAD_TLS. This brings us one step closer to getting rid of
> the copy_thread()/copy_thread_tls() split we still have and ultimately
> the HAVE_COPY_THREAD_TLS define in general. A lot of architectures have
> already converted and ia64 is one of the few hat haven't yet. This also
> unblocks implementing the clone3() syscall on ia64. Once that is done we
> can get of another ARCH_WANTS_* macro.
> 
> Once Any architecture that supports HAVE_COPY_THREAD_TLS cannot call the
> do_fork() helper anymore. This is fine and intended since it should be
> removed in favor of the new, cleaner _do_fork() calling convention based
> on struct kernel_clone_args. In fact, most architectures have already
> switched.  With this patch, ia64 joins the other arches which can't use
> the fork(), vfork(), clone(), clone3() syscalls directly and who follow
> the new process creation calling convention that is based on struct
> kernel_clone_args which we introduced a while back. This means less
> custom assembly in the architectures entry path to set up the registers
> before calling into the process creation helper and it is easier to to
> support new features without having to adapt calling conventions. It
> also unifies all process creation paths between fork(), vfork(),
> clone(), and clone3(). (We can't fix the ABI nightmare that legacy
> clone() is but we can prevent stuff like this happening in the future.)
> 
> Well, the first version I nothing to test this with. I don't know how to
> reasonably explain what happened but thanks to Adrian I'm now sitting at
> home next to a HP Integrity RX2600. I've done some testing and my initial
> version had a bug that became obvious when I took a closer look. The switch
> stack logic assumes that ar.pfs is stored in r16 and I changed that to r2.
> So with that fixed the following test program runs without any problems:
> 
>  #ifndef _GNU_SOURCE
>  #define _GNU_SOURCE 1
>  #endif
>  #include <errno.h>
>  #include <fcntl.h>
>  #include <linux/sched.h>
>  #include <sched.h>
>  #include <signal.h>
>  #include <stdbool.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
>  #include <sys/stat.h>
>  #include <sys/types.h>
>  #include <sys/wait.h>
>  #include <syscall.h>
>  #include <unistd.h>
> 
>  #define IA64_SYSCALL_OFFSET 1024
>  #ifndef __NR_clone
>  #define __NR_clone (104 + IA64_SYSCALL_OFFSET)
>  #endif
> 
>  #ifndef __NR_clone2
>  #define __NR_clone2 (189 + IA64_SYSCALL_OFFSET)
>  #endif
> 
>  /*
>   * sys_clone(unsigned long flags,
>   *	     unsigned long stack,
>   *	     int *parent_tidptr,
>   *	     int *child_tidptr,
>   *	     unsigned long tls)
>   */
>  static pid_t ia64_raw_clone(void)
>  {
>  	return syscall(__NR_clone, SIGCHLD, 0, NULL, NULL, 0);
>  }
> 
>  /*
>   * sys_clone2(unsigned long flags,
>   *	      unsigned long stack,
>   *	      unsigned long stack_size,
>   *	      int *parent_tidptr,
>   *	      int *child_tidptr,
>   *	      unsigned long tls)
>   */
>  static pid_t ia64_raw_clone2(void)
>  {
>  	return syscall(__NR_clone2, SIGCHLD, 0, 0, NULL, NULL, 0);
>  }
> 
>  /*
>   * Let's use the "standard stack limit" (i.e. glibc thread size default) for
>   * stack sizes: 8MB.
>   */
>  #define __STACK_SIZE (8 * 1024 * 1024)
> 
>  /* This is not always defined in sched.h. */
>  extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
>                       size_t __child_stack_size, int __flags, void *__arg, ...);
> 
>  pid_t libc_clone2(int (*fn)(void *), void *arg)
>  {
>  	pid_t ret;
>  	void *stack;
> 
>  	stack = malloc(__STACK_SIZE);
>  	if (!stack)
>  		return -ENOMEM;
> 
>  	 return __clone2(fn, stack, __STACK_SIZE, SIGCHLD, arg, NULL, NULL, NULL);
>  }
> 
>  static int libc_clone2_child(void *data)
>  {
>  	fprintf(stderr, "I'll just see myself out\n");
>  	_exit(EXIT_SUCCESS);
>  }
> 
>  int main(void)
>  {
>  	for (int i = 0; i < 1000; i++) {
>  		pid_t pid = ia64_raw_clone();
>  		if (pid < 0)
>  			_exit(EXIT_FAILURE);
> 
>  		if (pid == 0)
>  			_exit(EXIT_SUCCESS);
> 
>  		if (wait(NULL) != pid)
>  			_exit(EXIT_FAILURE);
>  		fprintf(stderr, "ia64_raw_clone() passed\n");
> 
>  		pid = ia64_raw_clone2();
>  		if (pid < 0)
>  			_exit(EXIT_FAILURE);
> 
>  		if (pid == 0)
>  			_exit(EXIT_SUCCESS);
> 
>  		if (wait(NULL) != pid)
>  			_exit(EXIT_FAILURE);
>  		fprintf(stderr, "ia64_raw_clone2() passed\n");
> 
>  		pid = libc_clone2(libc_clone2_child, NULL);
>  		if (pid < 0)
>  			_exit(EXIT_FAILURE);
> 
>  		if (wait(NULL) != pid)
>  			_exit(EXIT_FAILURE);
>  		fprintf(stderr, "libc_clone2() passed\n");
>  	}
> 
>  	_exit(EXIT_SUCCESS);
>  }
> 
> For some more context, please see:
> commit 606e9ad20094f6d500166881d301f31a51bc8aa7
> Merge: ac61145a725a 457677c70c76
> Author: Linus Torvalds <torvalds@...ux-foundation.org>
> Date:   Sat Jan 11 15:33:48 2020 -0800
> 
>     Merge tag 'clone3-tls-v5.5-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
> 
>     Pull thread fixes from Christian Brauner:
>      "This contains a series of patches to fix CLONE_SETTLS when used with
>       clone3().
> 
>       The clone3() syscall passes the tls argument through struct clone_args
>       instead of a register. This means, all architectures that do not
>       implement copy_thread_tls() but still support CLONE_SETTLS via
>       copy_thread() expecting the tls to be located in a register argument
>       based on clone() are currently unfortunately broken. Their tls value
>       will be garbage.
> 
>       The patch series fixes this on all architectures that currently define
>       __ARCH_WANT_SYS_CLONE3. It also adds a compile-time check to ensure
>       that any architecture that enables clone3() in the future is forced to
>       also implement copy_thread_tls().
> 
>       My ultimate goal is to get rid of the copy_thread()/copy_thread_tls()
>       split and just have copy_thread_tls() at some point in the not too
>       distant future (Maybe even renaming copy_thread_tls() back to simply
>       copy_thread() once the old function is ripped from all arches). This
>       is dependent now on all arches supporting clone3().
> 
>       While all relevant arches do that now there are still four missing:
>       ia64, m68k, sh and sparc. They have the system call reserved, but not
>       implemented. Once they all implement clone3() we can get rid of
>       ARCH_WANT_SYS_CLONE3 and HAVE_COPY_THREAD_TLS.
> 
> Note that in the meantime, m68k has already switched to the new calling
> convention. And I've got sparc patches acked by Dave, too.
> 
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Qais Yousef <qais.yousef@....com>
> Cc: linux-ia64@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> /* v2 */
> - Christian Brauner <christian.brauner@...ntu.com>:
>   - Continue to preserve afs.pfs in r16. I wasn't clear that r16 needs to
>     be used because switch stack and load stack rely on it being saved in
>     r16 and they'll be very unhappy when it's not. r16 is clobbered though
>     so now the mov loc1=r16 in there makes sense to me.
>   - Well, it's tested now...

Tony, I managed to test this now.

Christian

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