Re: [RFC PATCH v2] waitfd

From: Randy Dunlap
Date: Tue Jan 06 2009 - 13:33:52 EST

Casey Dahlin wrote:
> Linux now exposes signals, timers, and events via file descriptors
> through signalfd, timerfd, and eventfd. This means programmers can use a
> single select/[e]poll call to monitor all change in their program. This
> patch aims to expose child death via the same mechanism.
>
> waitfd provides a file descriptor out of which may be read a series of
> siginfo_t objects describing child death. A child process is reaped as
> soon as its information is read. This means child monitoring too can be
> performed with that same poll call.
>
> Patch is against v2.6.28
>
> --CJD
>
> diff --git a/arch/x86/include/asm/unistd_32.h
> b/arch/x86/include/asm/unistd_32.h
> index f2bba78..134d83c 100644
> --- a/arch/x86/include/asm/unistd_32.h
> +++ b/arch/x86/include/asm/unistd_32.h
> @@ -338,6 +338,7 @@
> #define __NR_dup3 330
> #define __NR_pipe2 331
> #define __NR_inotify_init1 332
> +#define __NR_waitfd 333
>
> #ifdef __KERNEL__
>
> diff --git a/arch/x86/include/asm/unistd_64.h
> b/arch/x86/include/asm/unistd_64.h
> index d2e415e..b28eb07 100644
> --- a/arch/x86/include/asm/unistd_64.h
> +++ b/arch/x86/include/asm/unistd_64.h
> @@ -653,6 +653,8 @@ __SYSCALL(__NR_dup3, sys_dup3)
> __SYSCALL(__NR_pipe2, sys_pipe2)
> #define __NR_inotify_init1 294
> __SYSCALL(__NR_inotify_init1, sys_inotify_init1)
> +#define __NR_waitfd 295
> +__SYSCALL(__NR_waitfd, sys_waitfd)
>

Only for x86??

>
> #ifndef __NO_STUBS
> diff --git a/arch/x86/kernel/syscall_table_32.S
> b/arch/x86/kernel/syscall_table_32.S
> index d44395f..c796a8b 100644
> --- a/arch/x86/kernel/syscall_table_32.S
> +++ b/arch/x86/kernel/syscall_table_32.S
> @@ -332,3 +332,4 @@ ENTRY(sys_call_table)
> .long sys_dup3 /* 330 */
> .long sys_pipe2
> .long sys_inotify_init1
> + .long sys_waitfd
> diff --git a/fs/Makefile b/fs/Makefile
> index d9f8afe..74c31fb 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -25,6 +25,7 @@ obj-$(CONFIG_INOTIFY_USER) += inotify_user.o
> obj-$(CONFIG_EPOLL) += eventpoll.o
> obj-$(CONFIG_ANON_INODES) += anon_inodes.o
> obj-$(CONFIG_SIGNALFD) += signalfd.o
> +obj-$(CONFIG_WAITFD) += waitfd.o
> obj-$(CONFIG_TIMERFD) += timerfd.o
> obj-$(CONFIG_EVENTFD) += eventfd.o
> obj-$(CONFIG_AIO) += aio.o
> diff --git a/fs/waitfd.c b/fs/waitfd.c
> new file mode 100644
> index 0000000..0155a83
> --- /dev/null
> +++ b/fs/waitfd.c
> @@ -0,0 +1,117 @@
> +/*
> + * fs/waitfd.c
> + *
> + * Copyright (C) 2008 Red Hat, Casey Dahlin <cdahlin@xxxxxxxxxx>
> + *
> + * Largely derived from fs/signalfd.c
> + */
> +
> +#include <linux/file.h>
> +#include <linux/poll.h>
> +#include <linux/init.h>
> +#include <linux/fs.h>
> +#include <linux/sched.h>
> +#include <linux/kernel.h>
> +#include <linux/signal.h>
> +#include <linux/list.h>
> +#include <linux/anon_inodes.h>
> +#include <linux/syscalls.h>
> +
> +long do_waitid(int which, pid_t upid,
> + struct siginfo __user *infop, int options,
> + struct rusage __user *ru);
> +
> +struct waitfd_ctx {
> + int ops;
> + int which;
> + pid_t upid;
> +};
> +

Please use kernel coding style: use tabs to indent, not <lots-of-spaces>,
and struct members, functions, etc., are indented by one tab stop minimum.

> +static int waitfd_release(struct inode *inode, struct file *file)
> +{
> + kfree(file->private_data);
> + return 0;
> +}
> +
> +static unsigned int waitfd_poll(struct file *file, poll_table *wait)
> +{
> + struct waitfd_ctx *ctx = file->private_data;
> + long value;
> +
> + poll_wait(file, &current->signal->wait_chldexit, wait);
> +
> + value = do_waitid(ctx->which, ctx->upid, NULL,
> + ctx->ops | WNOHANG | WNOWAIT, NULL);
> + if (value > 0 || value == -ECHILD)
> + return POLLIN;
> +
> + return 0;
> +}
> +
> +/*
> + * Returns a multiple of the size of a struct siginfo, or a negative
> + * error code. The "count" parameter must be at least sizeof(struct
> siginfo)
> + */
> +static ssize_t waitfd_read(struct file *file, char __user *buf, size_t
> count,
> + loff_t *ppos)
> +{
> + struct waitfd_ctx *ctx = file->private_data;
> + struct siginfo __user *info_addr = (struct siginfo *)buf;
> + int flags = ctx->ops;
> + ssize_t ret, total = 0;
> +
> + count /= sizeof(struct siginfo);
> + if (!count)
> + return -EINVAL;
> +
> + do {
> + ret = do_waitid(ctx->which, ctx->upid, info_addr, flags, NULL);
> + if (ret == 0)
> + ret = -EAGAIN;
> + if (ret == -ECHILD)
> + ret = 0;
> + if (ret <= 0)
> + break;
> +
> + info_addr++;
> + total += sizeof(struct siginfo);
> + flags |= WNOHANG;
> + } while (--count);
> +
> + return total ? total: ret;

return total ? total : ret;
please.

> +}
> +
> +static const struct file_operations waitfd_fops = {
> + .release = waitfd_release,
> + .poll = waitfd_poll,
> + .read = waitfd_read,
> +};
> +
> +asmlinkage long sys_waitfd(int which, pid_t upid, int options, int unused)
> +{
> + int ufd;
> + struct waitfd_ctx *ctx;
> +
> + /* Just to make sure we don't end up with a sys_waitfd4 */
> + (void)unused;
> +
> + if (options & ~(WNOHANG|WEXITED|WSTOPPED|WCONTINUED))
> + return -EINVAL;
> + if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
> + return -EINVAL;

Use spaces around '|'.

> +
> + ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
> + if (!ctx)
> + return -ENOMEM;
> +
> + ctx->ops = options;
> + ctx->upid = upid;
> + ctx->which = which;
> +
> + ufd = anon_inode_getfd("[waitfd]", &waitfd_fops, ctx,
> + (options & WNOHANG) ? O_NONBLOCK : 0);
> + if (ufd < 0)
> + kfree(ctx);
> +
> + return ufd;
> +}
> diff --git a/init/Kconfig b/init/Kconfig
> index f763762..bc34871 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -683,6 +683,16 @@ config EPOLL
> Disabling this option will cause the kernel to be built without
> support for epoll family of system calls.
>
> +config WAITFD
> + bool "Enable waitfd() system call" if EMBEDDED
> + select ANON_INODES
> + default y
> + help
> + Enable the waitfd() system call that allows receving child state

receiving

Kconfig help text should be indented by <tab><space><space>.

> + changes from a file descriptor.
> +
> + If unsure, say Y.
> +
> config SIGNALFD
> bool "Enable signalfd() system call" if EMBEDDED
> select ANON_INODES
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 2d8be7e..b53e8ba 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1233,18 +1233,20 @@ static int wait_noreap_copyout(struct
> task_struct *p, pid_t pid, uid_t uid,
> int retval = rusagep ? getrusage(p, RUSAGE_BOTH, rusagep) : 0;
>
> put_task_struct(p);
> - if (!retval)
> - retval = put_user(SIGCHLD, &infop->si_signo);
> - if (!retval)
> - retval = put_user(0, &infop->si_errno);
> - if (!retval)
> - retval = put_user((short)why, &infop->si_code);
> - if (!retval)
> - retval = put_user(pid, &infop->si_pid);
> - if (!retval)
> - retval = put_user(uid, &infop->si_uid);
> - if (!retval)
> - retval = put_user(status, &infop->si_status);
> + if (infop) {
> + if (!retval)
> + retval = put_user(SIGCHLD, &infop->si_signo);
> + if (!retval)
> + retval = put_user(0, &infop->si_errno);
> + if (!retval)
> + retval = put_user((short)why, &infop->si_code);
> + if (!retval)
> + retval = put_user(pid, &infop->si_pid);
> + if (!retval)
> + retval = put_user(uid, &infop->si_uid);
> + if (!retval)
> + retval = put_user(status, &infop->si_status);
> + }
> if (!retval)
> retval = pid;
> return retval;
> @@ -1727,35 +1729,12 @@ repeat:
> end:
> current->state = TASK_RUNNING;
> remove_wait_queue(&current->signal->wait_chldexit,&wait);
> - if (infop) {
> - if (retval > 0)
> - retval = 0;
> - else {
> - /*
> - * For a WNOHANG return, clear out all the fields
> - * we would set so the user can easily tell the
> - * difference.
> - */
> - if (!retval)
> - retval = put_user(0, &infop->si_signo);
> - if (!retval)
> - retval = put_user(0, &infop->si_errno);
> - if (!retval)
> - retval = put_user(0, &infop->si_code);
> - if (!retval)
> - retval = put_user(0, &infop->si_pid);
> - if (!retval)
> - retval = put_user(0, &infop->si_uid);
> - if (!retval)
> - retval = put_user(0, &infop->si_status);
> - }
> - }
> return retval;
> }
>
> -asmlinkage long sys_waitid(int which, pid_t upid,
> - struct siginfo __user *infop, int options,
> - struct rusage __user *ru)
> +long do_waitid(int which, pid_t upid,
> + struct siginfo __user *infop, int options,
> + struct rusage __user *ru)
> {
> struct pid *pid = NULL;
> enum pid_type type;
> @@ -1789,6 +1768,39 @@ asmlinkage long sys_waitid(int which, pid_t upid,
> ret = do_wait(type, pid, options, infop, NULL, ru);
> put_pid(pid);
>
> + return ret;
> +}
> +
> +asmlinkage long sys_waitid(int which, pid_t upid,
> + struct siginfo __user *infop, int options,
> + struct rusage __user *ru)
> +{
> + long ret;
> +
> + ret = do_waitid(which, upid, infop, options, ru);
> +
> + if (ret > 0)
> + ret = 0;
> + else {
> + /*
> + * For a WNOHANG return, clear out all the fields
> + * we would set so the user can easily tell the
> + * difference.
> + */
> + if (!ret)
> + ret = put_user(0, &infop->si_signo);
> + if (!ret)
> + ret = put_user(0, &infop->si_errno);
> + if (!ret)
> + ret = put_user(0, &infop->si_code);
> + if (!ret)
> + ret = put_user(0, &infop->si_pid);
> + if (!ret)
> + ret = put_user(0, &infop->si_uid);
> + if (!ret)
> + ret = put_user(0, &infop->si_status);
> + }
> +
> /* avoid REGPARM breakage on x86: */
> asmlinkage_protect(5, ret, which, upid, infop, options, ru);
> return ret;
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index e14a232..e8d4da6 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -163,6 +163,7 @@ cond_syscall(sys_ioprio_set);
> cond_syscall(sys_ioprio_get);
>
> /* New file descriptors */
> +cond_syscall(sys_waitfd);
> cond_syscall(sys_signalfd);
> cond_syscall(sys_signalfd4);
> cond_syscall(compat_sys_signalfd);


--
~Randy
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