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SubjectRe: [patch] threading enhancements, tid-2.5.47-C0

On 18 Nov 2002, Luca Barbieri wrote:

> How about making ->set_child_tid a parameter for schedule_tail, or even
> directly using it in the ret_from_fork assembly? It doesn't make much
> sense to have a variable in task_struct which is used only at task
> creation.

what we could do is in fact to make the child address synchronous, see the
attached patch.

this moves the complexity of setting the child TID from the kernel into
glibc. glibc's fork() implementation would have to set up a new thread
descriptor for the child (which would be served from the stack-cache most
of the time so it's not significant overhead), because the child TID is
also set in the parent's VM, but otherwise it's good enough.

the even simpler solution would be to keep things as-is in 2.5.48, this
would remove the ability of cfork()-alike functionality. glibc would still
have to allocate a new descriptor in the fork() case - but compared to the
generic overhead of fork() this should not be an issue.

Ingo

--- linux/arch/i386/kernel/smpboot.c.orig 2002-11-17 21:12:49.000000000 +0100
+++ linux/arch/i386/kernel/smpboot.c 2002-11-17 21:12:52.000000000 +0100
@@ -498,7 +498,7 @@
* don't care about the eip and regs settings since
* we'll never reschedule the forked task.
*/
- return do_fork(CLONE_VM|CLONE_IDLETASK, 0, &regs, 0, NULL);
+ return do_fork(CLONE_VM|CLONE_IDLETASK, 0, &regs, 0, NULL, NULL);
}

/* which physical APIC ID maps to which logical CPU number */
--- linux/arch/i386/kernel/process.c.orig 2002-11-17 21:03:01.000000000 +0100
+++ linux/arch/i386/kernel/process.c 2002-11-18 12:51:31.000000000 +0100
@@ -225,7 +225,7 @@
regs.eflags = 0x286;

/* Ok, create the new process.. */
- p = do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL);
+ p = do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
return IS_ERR(p) ? PTR_ERR(p) : p->pid;
}

@@ -287,7 +287,7 @@
struct_cpy(childregs, regs);
childregs->eax = 0;
childregs->esp = esp;
- p->user_tid = NULL;
+ p->clear_child_tid = NULL;

p->thread.esp = (unsigned long) childregs;
p->thread.esp0 = (unsigned long) (childregs+1);
@@ -502,7 +502,7 @@
{
struct task_struct *p;

- p = do_fork(SIGCHLD, regs.esp, &regs, 0, NULL);
+ p = do_fork(SIGCHLD, regs.esp, &regs, 0, NULL, NULL);
return IS_ERR(p) ? PTR_ERR(p) : p->pid;
}

@@ -511,14 +511,15 @@
struct task_struct *p;
unsigned long clone_flags;
unsigned long newsp;
- int *user_tid;
+ int *parent_tidptr, *child_tidptr;

clone_flags = regs.ebx;
newsp = regs.ecx;
- user_tid = (int *)regs.edx;
+ parent_tidptr = (int *)regs.edx;
+ child_tidptr = (int *)regs.esi;
if (!newsp)
newsp = regs.esp;
- p = do_fork(clone_flags & ~CLONE_IDLETASK, newsp, &regs, 0, user_tid);
+ p = do_fork(clone_flags & ~CLONE_IDLETASK, newsp, &regs, 0, parent_tidptr, child_tidptr);
return IS_ERR(p) ? PTR_ERR(p) : p->pid;
}

@@ -536,7 +537,7 @@
{
struct task_struct *p;

- p = do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0, NULL);
+ p = do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0, NULL, NULL);
return IS_ERR(p) ? PTR_ERR(p) : p->pid;
}

--- linux/include/linux/sched.h.orig 2002-11-17 20:53:52.000000000 +0100
+++ linux/include/linux/sched.h 2002-11-18 12:48:26.000000000 +0100
@@ -46,10 +46,11 @@
#define CLONE_NEWNS 0x00020000 /* New namespace group? */
#define CLONE_SYSVSEM 0x00040000 /* share system V SEM_UNDO semantics */
#define CLONE_SETTLS 0x00080000 /* create a new TLS for the child */
-#define CLONE_SETTID 0x00100000 /* write the TID back to userspace */
-#define CLONE_CLEARTID 0x00200000 /* clear the userspace TID */
-#define CLONE_DETACHED 0x00400000 /* parent wants no child-exit signal */
-#define CLONE_UNTRACED 0x00800000 /* set if the tracing process can't force CLONE_PTRACE on this clone */
+#define CLONE_PARENT_SETTID 0x00100000 /* set the TID in the parent */
+#define CLONE_CHILD_CLEARTID 0x00200000 /* clear the TID in the child */
+#define CLONE_DETACHED 0x00400000 /* parent wants no child-exit signal */
+#define CLONE_UNTRACED 0x00800000 /* set if the tracing process can't force CLONE_PTRACE on this clone */
+#define CLONE_CHILD_SETTID 0x01000000 /* set the TID in the child */

/*
* List of flags we want to share for kernel threads,
@@ -332,7 +333,7 @@

wait_queue_head_t wait_chldexit; /* for wait4() */
struct completion *vfork_done; /* for vfork() */
- int *user_tid; /* for CLONE_CLEARTID */
+ int *clear_child_tid; /* CLONE_CHILD_CLEARTID */

unsigned long rt_priority;
unsigned long it_real_value, it_prof_value, it_virt_value;
@@ -615,7 +616,7 @@
extern task_t *child_reaper;

extern int do_execve(char *, char **, char **, struct pt_regs *);
-extern struct task_struct *do_fork(unsigned long, unsigned long, struct pt_regs *, unsigned long, int *);
+extern struct task_struct *do_fork(unsigned long, unsigned long, struct pt_regs *, unsigned long, int *, int *);

#ifdef CONFIG_SMP
extern void wait_task_inactive(task_t * p);
--- linux/kernel/fork.c.orig 2002-11-17 20:54:55.000000000 +0100
+++ linux/kernel/fork.c 2002-11-18 12:50:11.000000000 +0100
@@ -407,13 +407,13 @@
tsk->vfork_done = NULL;
complete(vfork_done);
}
- if (tsk->user_tid) {
+ if (tsk->clear_child_tid) {
/*
* We dont check the error code - if userspace has
* not set up a proper pointer then tough luck.
*/
- put_user(0, tsk->user_tid);
- sys_futex((unsigned long)tsk->user_tid, FUTEX_WAKE, 1, NULL);
+ put_user(0, tsk->clear_child_tid);
+ sys_futex((unsigned long)tsk->clear_child_tid, FUTEX_WAKE, 1, NULL);
}
}

@@ -676,9 +676,9 @@
p->flags = new_flags;
}

-asmlinkage int sys_set_tid_address(int *user_tid)
+asmlinkage int sys_set_tid_address(int *tidptr)
{
- current->user_tid = user_tid;
+ current->clear_child_tid = tidptr;

return current->pid;
}
@@ -695,7 +695,8 @@
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
- int *user_tid)
+ int *parent_tidptr,
+ int *child_tidptr)
{
int retval;
struct task_struct *p = NULL;
@@ -819,19 +820,19 @@
retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
if (retval)
goto bad_fork_cleanup_namespace;
- /*
- * Notify the child of the TID?
- */
+
retval = -EFAULT;
- if (clone_flags & CLONE_SETTID)
- if (put_user(p->pid, user_tid))
+ if (clone_flags & CLONE_PARENT_SETTID)
+ if (put_user(p->pid, parent_tidptr))
+ goto bad_fork_cleanup_namespace;
+ if (clone_flags & CLONE_CHILD_SETTID)
+ if (put_user(p->pid, child_tidptr))
goto bad_fork_cleanup_namespace;
-
/*
- * Does the userspace VM want the TID cleared on mm_release()?
+ * Clear TID on mm_release()?
*/
- if (clone_flags & CLONE_CLEARTID)
- p->user_tid = user_tid;
+ if (clone_flags & CLONE_CHILD_CLEARTID)
+ p->clear_child_tid = child_tidptr;

/*
* Syscall tracing should be turned off in the child regardless
@@ -1000,7 +1001,8 @@
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
- int *user_tid)
+ int *parent_tidptr,
+ int *child_tidptr)
{
struct task_struct *p;
int trace = 0;
@@ -1011,7 +1013,7 @@
clone_flags |= CLONE_PTRACE;
}

- p = copy_process(clone_flags, stack_start, regs, stack_size, user_tid);
+ p = copy_process(clone_flags, stack_start, regs, stack_size, parent_tidptr, child_tidptr);
if (!IS_ERR(p)) {
struct completion vfork;

-
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