lkml.org 
[lkml]   [1998]   [Dec]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: Is this the right list?
Date
> synchronization mechanisms I am using.  When is it safe and not safe to
> use the start_bh_atomic() and end_bh_atomic() calls to try to guarantee

A bottom half handler (where is already implicit as bh's are currently
atomic w.r.t each other and themselves), and user space where it will
prevent a BH from executing during the marked section (and on SMP in 2.1.x
wait for a BH to complete - since a BH could be running on another CPU
in parallel in 2.1.x but not 2.0.x).

> I roll my own? Also, when I look at the defs for the
> start/end_bh_atomic() calls, they reference a function called barrier()
> that is #defined as:
>
> #define barrier() __asm__("": : :"memory")

The instruction is "" (ie nothing) and the "memory" is the thing it affects
- so barrier is a null instruction that arbitarily affects memory space.

Its used to persuade gcc to write back and invalidate any register temporary
variables

Without it something like

start_bh_atomic();
x=list->head;
list->head=list->head->next;
end_bh_atomic();

might be re-ordered by the compiler either so the x=list->head moves before
the beginning of the atomic area or the list->head= is moved past the end.
Its a better alternative than using volatile in most cases

Alan


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:45    [W:0.036 / U:23.500 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site