On Dec 16, 2005, at 10:24, Linh Dang wrote:
Well, it may be true for ARM but for ppc (i dunno what exactly llsc
means but someone in the thread put ppc in llsc group) it's:
loop:
load-reserve foo => old
new = old * N
store-conditional new => foo
if failed goto loop
LLSC == Load-Locked/Store-Conditional. It's a slightly different
name for your Load-Reserve/Store-Conditional
You still miss his point. That is _GOOD_ code. Russell's point is
that if somebody does this in generic code:
do {
old = atomic_read(&foo);
new = old * 2;
} while (atomic_cmpxchg(&foo, old, new) != old);
On PPC or ARM or another LLSC architecture it does not end up looking
like the good code, it looks like this (which is clearly inefficient):
And for architectures with llsc, this becomes:
loop: load foo => old
new = old * N
loop2: load locked foo => ret
compare ret & old
if equal store conditional new in foo
if store failed because we lost the lock, goto loop2
compare ret & old
if not equal goto loop
Cheers,
Kyle Moffett
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