Satyam Sharma wrote:
On Tue, 14 Aug 2007, Christoph Lameter wrote:
On Thu, 9 Aug 2007, Chris Snook wrote:
This patchset makes the behavior of atomic_read uniform by removing the
volatile keyword from all atomic_t and atomic64_t definitions that currently
have it, and instead explicitly casts the variable as volatile in
atomic_read(). This leaves little room for creative optimization by the
compiler, and is in keeping with the principles behind "volatile considered
harmful".
volatile is generally harmful even in atomic_read(). Barriers control
visibility and AFAICT things are fine.
Frankly, I don't see the need for this series myself either. Personal
opinion (others may differ), but I consider "volatile" to be a sad /
unfortunate wart in C (numerous threads on this list and on the gcc
lists/bugzilla over the years stand testimony to this) and if we _can_
steer clear of it, then why not -- why use this ill-defined primitive
whose implementation has often differed over compiler versions and
platforms? Granted, barrier() _is_ heavy-handed in that it makes the
optimizer forget _everything_, but then somebody did post a forget()
macro on this thread itself ...
[ BTW, why do we want the compiler to not optimize atomic_read()'s in
the first place? Atomic ops guarantee atomicity, which has nothing
to do with "volatility" -- users that expect "volatility" from
atomic ops are the ones who must be fixed instead, IMHO. ]
Because atomic operations are generally used for synchronization, which requires
volatile behavior. Most such codepaths currently use an inefficient barrier().
Some forget to and we get bugs, because people assume that atomic_read()
actually reads something, and atomic_write() actually writes something. Worse,
these are architecture-specific, even compiler version-specific bugs that are
often difficult to track down.
-- Chris
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