[lkml]   [2003]   [Aug]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Lockless file reading
Richard B. Johnson wrote:
> Let's see if it is possible for the middle byte of
> a 3-byte sequence to not be written when both
> other bytes are written:

> Even in machines that do load/store operations where individual
> components of those stores happen in groups, access to/from
> a buffer of such data is controlled (by hardware) so a write
> will complete before a read occurs.

I don't understand what you mean by this.

Do you mean that the writes are forced to appear on a different CPU in
the same order that they were written? That isn't true on x86, for
two reasons: 1. writes aren't always in processor order (see
CONFIG_X86_OOSTORE and CONFIG_X86_PPRO_FENCE); 2. reads on the other
processor are out of order anyway.

> With hardware that can perform byte-access (ix86), the only
> byte-access that is going to happen is at the end(s) of buffers.

Not true. Take a look at __copy_user() in arch/i386/lib/usercopy.c.
The first few bytes are copied using "rep;movsb", which is not
guaranteed to use a word write for the aligned pair, nor is it
guaranteed a particular timing (there could be an interrupt between
each byte).

Other architectures are similar.

-- Jamie
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 13:48    [W:0.260 / U:1.564 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site