On Mon, Feb 09, 2015 at 05:00:49PM +1100, Bruce Evans wrote: > On Mon, 9 Feb 2015, Oleksandr Tymoshenko wrote: > > > Log: > > Do not mark shared structures as __packed, it leads to race condition > > > > If structure packed as __packed clang (and probably gcc) generates > > code that loads word fields (e.g. tx_pos) byte-by-byte and if it's > > modified by VideoCore in the same time as ARM loads the value result > > is going to be mixed combination of bytes from previous value and > > new one. > > Most uses of __packed are bugs. It gives pessimizations as well as > non-atomic accesses for sub-object accesses. > > I think the full bugs only occur when arch has strict alignment > requirements and the alignment of the __packed objects is not known. > This means that only lesser bugs occur on x86 (unless you enable > alignment checking, but this arguably breaks the ABI). The compiler > just generates possibly-misaligned full-width accesses if the arch > doesn't have strict alignment requirements. Often the acceses turn > out to be aligned at runtime. Otherwise, the hardware does them > atomically, with a smaller efficiency penalty than split accesses.
On x86 unaligned access is non-atomic. This was very visible on Core2 CPUs where DPCPU code mishandled the alignment, resulting in the mutexes from the per-cpu areas breaking badly. Modern CPUs should not lock several cache lines simultaneously either. _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
