On Thu, 31 Jan 2002, Maciej W. Rozycki wrote: > Date: Thu, 31 Jan 2002 23:17:21 +0100 (MET) > From: Maciej W. Rozycki <macro@ds2.pg.gda.pl> > To: H . J . Lu <hjl@lucon.org> > Cc: GNU C Library <libc-alpha@sources.redhat.com>, linux-mips@oss.sgi.com > Subject: [libc-alpha] Re: PATCH: Fix ll/sc for mips > > On Thu, 31 Jan 2002, H . J . Lu wrote: > > > (__compare_and_swap): Return 0 when failed to compare or swap. > [...] > > * sysdeps/mips/atomicity.h (compare_and_swap): Return 0 when > > failed to compare or swap. > > Looking at the i486 implementation these are not expected to fail. > Unless I am missing something... That's what ``compare'' means in ``compare and swap''. You lock the memory location at some hardware level and then compare the location to the specified value. If there is a match, you change the memory location to the new value. Otherwise you don't, and indicate that you didn't. That's what ``fail'' means. The comparison failed, and the operation failed to install the new value. The caller must detect this and handle that. Algorithms based on atomic compare-and-swap take failure into account.
