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SubjectRe: [PATCHv11 2.6.36-rc2-tip 3/15] 3: uprobes: Slot allocation for Execution out of line(XOL)
> > 
> > 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;
> > }
>
> Racy like you won't believe..
>
> Suppose multiple threads hitting the trap at the same time, every thread
> will end up failing the check and allocating a new slot for it, at the
> end the slowest thread will end up setting the value.
>

Agree, I shall fix this up.
Since set_bit and clear_bit are atomic, I shall change the
area->lock from a spinlock to a mutex, and have the mutex released
after the slot has been updated with the "single-stepping
instruction".

--
Thanks and Regards
Srikar


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