On Saturday, 12 May 2007 01:25, Andrew Morton wrote:
> On Sat, 12 May 2007 01:22:06 +0200
> "Rafael J. Wysocki" <[EMAIL PROTECTED]> wrote:
>
> > On Saturday, 12 May 2007 00:56, Linus Torvalds wrote:
> > >
> > > On Fri, 11 May 2007, Rafael J. Wysocki wrote:
> > > >
> > > > For user space processes this condition is always true.
> > > >
> > > > For kernel threads:
> > > > (1) the change of tsk->mm from NULL to a nonzero value is only made in
> > > > fs/aio.c:use_mm() along with the setting of PF_BORROWED_MM under
> > > > the task_lock(),
> > > > (2) the change of tsk->mm from a nonzero value to NULL is only made in
> > > > fs/aio.c:unuse_mm() along with the resetting of PF_BORROWED_MM
> > > > under the task_lock().
> > > > Therefore, by taking the task_lock() here we make sure that the
> > > > condition
> > > > is alyways false when we check it for kernel threads.
> > >
> > > Why *test* it then and return anything?
> > >
> > > Why not just doa "task_lock(p); task_unlock(p);" with no return value?
> > >
> > > As it is, it sounds like either the code is buggy, or it's pointless.
> >
> > I'm not sure what you mean.
> >
> > We use this function (ie. kernel/power/process.c:is_user_space()) to
> > distinguish kernel threads from user space processes. Therefore we make it
> > always return true for user space processes and always return false for
> > kernel
> > threads. In the latter case we need to use the task_lock() to ensure that
> > the
> > result is as desired (ie. false), because otherwise it might be racing with
> > either fs/aio.c:use_mm() or fs/aio.c:unuse_mm().
> >
>
> ah, OK.
>
> static void use_mm(struct mm_struct *mm)
> {
> struct mm_struct *active_mm;
> struct task_struct *tsk = current;
>
> task_lock(tsk);
> tsk->flags |= PF_BORROWED_MM;
> active_mm = tsk->active_mm;
> atomic_inc(&mm->mm_count);
> tsk->mm = mm;
> tsk->active_mm = mm;
> /*
> * Note that on UML this *requires* PF_BORROWED_MM to be set, otherwise
> * it won't work. Update it accordingly if you change it here
> */
> switch_mm(active_mm, mm, tsk);
> task_unlock(tsk);
>
> So is_user_space() requires that the state of p->mm and p->flags be
> consistent: it doesn't want to be looking at those two things in that
> three-statement window above.
>
> Good changelogging and commenting save quite a bit of time and email.
Very true.
I have added a comment to the patch, so that we remeber why the task_lock()
is there. Please replace the original patch with this one (unless you think
it's
worse ;-)).
---
From: Rafael J. Wysocki <[EMAIL PROTECTED]>
The reading of PF_BORROWED_MM in is_user_space() without task_lock() is racy.
Fix it.
Signed-off-by: Rafael J. Wysocki <[EMAIL PROTECTED]>
Acked-by: Pavel Machek <[EMAIL PROTECTED]>
---
kernel/power/process.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
Index: linux-2.6/kernel/power/process.c
===================================================================
--- linux-2.6.orig/kernel/power/process.c
+++ linux-2.6/kernel/power/process.c
@@ -8,6 +8,7 @@
#undef DEBUG
+#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/suspend.h>
#include <linux/module.h>
@@ -88,7 +89,18 @@ static void cancel_freezing(struct task_
static inline int is_user_space(struct task_struct *p)
{
- return p->mm && !(p->flags & PF_BORROWED_MM);
+ int ret;
+
+ /*
+ * task_lock() is acquired to avoid evaluating the condition while the
+ * state of p->mm and p->flags is not consistent, which may happen,
+ * for example, if this function is executed in parallel with
+ * fs/aio.c:unuse_mm()
+ */
+ task_lock(p);
+ ret = p->mm && !(p->flags & PF_BORROWED_MM);
+ task_unlock(p);
+ return ret;
}
static unsigned int try_to_freeze_tasks(int freeze_user_space)
-
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