lkml.org 
[lkml]   [2010]   [Sep]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCHv11 2.6.36-rc2-tip 3/15] 3: uprobes: Slot allocation for Execution out of line(XOL)
* Peter Zijlstra <peterz@infradead.org> [2010-09-03 18:51:54]:

> On Fri, 2010-09-03 at 22:10 +0530, Srikar Dronamraju wrote:
> > > > + mb();
> > >
> > > Where is the matching barrier?
> >
> > I dont want the compiler to reorder the instructions and do the
> > assignment for user_bkpt to be done before we complete the copy above.
> >
> > If the assignment happens before we copy the content into the slot,
> > someother thread that might hit the same probe actually things the slot
> > is ready and tries to jump to that slot even before the slot is
> > initialized.
> >
> > Please let me know if I could have done it differently.
>
>
> If you want a compiler barrier, use barrier(), but here you seem to
> describe a multi-threaded situation, in which case the observer thread
> needs at least a rmb() in order for that mb() to mean anything other
> than the compiler barrier it implies.
>
> Also, use smp_* barriers.
>
>
>

Okay, would something like this suffice?


static unsigned long xol_get_insn_slot(struct user_bkpt *user_bkpt,
struct uprobes_xol_area *xol_area)
{
unsigned long flags, xol_vaddr = 0;
int len;

if (unlikely(!xol_area))
return 0;

smp_rmb();
if (user_bkpt->xol_vaddr)
return user_bkpt->xol_vaddr;

spin_lock_irqsave(&xol_area->lock, flags);
xol_vaddr = xol_take_insn_slot(xol_area);
spin_unlock_irqrestore(&xol_area->lock, flags);

/*
* Initialize the slot if user_bkpt->vaddr points to valid
* instruction slot.
*/
if (!xol_vaddr)
return 0;

len = access_process_vm(current, xol_vaddr, user_bkpt->insn,
UPROBES_XOL_SLOT_BYTES, 1);
if (unlikely(len < UPROBES_XOL_SLOT_BYTES))
printk(KERN_ERR "Failed to copy instruction at %#lx "
"len = %d\n", user_bkpt->vaddr, len);

/*
* Update user_bkpt->xol_vaddr after giving a chance for the slot to
* be initialized.
*/
smp_mb();
user_bkpt->xol_vaddr = xol_vaddr;
return user_bkpt->xol_vaddr;
}

--
Thanks and Regards
Srikar


\
 
 \ /
  Last update: 2010-09-03 19:35    [W:0.079 / U:40.092 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site