help with access_process_vm

[Fabiano Ramos <fabiano.ramos@gmail.com>]

Hi all.

I am trying to write/read to/from a process image (to be more
specific, replace an instruction with a 0xCC trap) from within a debug
handler.  I mean, a debug handler will be fired
(via eflags) and I want to make sure the process will stop again at a
given address.

My new handler, called do_debug_new replaces the old do_debug. From inside it,
I do something like

       task = current;
       ....
       access_process_vm(task, addr, &oldvalue, sizeof(oldvalue), 0);
       newvalue= oldvalue;
       ptr = (char *) &newvalue;
       *ptr = 0xCC; 
       access_process_vm(task, addr, &newvalue, sizeof(newvalue), 1);
        ....

But the first time a call access_process_vm, dmesg shows me:
     
Debug: sleeping function called from invalid context at include/linux/rwsem.h:43
in_atomic():0, irqs_disabled():1
 [<c01145ac>] __might_sleep+0x8c/0xa0
 [<c011c69b>] access_process_vm+0x4b/0x1d0
 [<c010c830>] do_debug_new+0xd0/0x190
 [<c038c755>] schedule+0x275/0x460
 [<c0105c2d>] error_code+0x2d/0x40

What I am missing? Do I need some syncronization? Can the debug
handler run in the
context of a process that was not the one that caused the debug trap?

Thanks a lot
Fabiano