On Sun, Jan 03, 2016 at 06:25:39PM +0200, Petko Manolov wrote: > Content preview: Hi Paul, Ingo, It seems to me that smp_read_barrier_depends > (which resolves to ___raw_smp_check_barrier_asm) is overdoing it, unless > that particular part it is written for is a disaster in terms of cache coherency. > [...] > > Content analysis details: (-1.0 points, 5.0 required) > > pts rule name description > ---- ---------------------- -------------------------------------------------- > -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP > X-ZLA-Header: unknown; 0 > X-ZLA-DetailInfo: BA=6.00004041; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZH=6.00000000; ZP=6.00000000; ZU=6.00000002; UDB=6.00287883; UTC=2016-01-03 16:24:57 > x-cbid: 16010316-2214-0000-0000-00000F04BD9C > X-IBM-ISS-SpamDetectors: Score=0.415652; FLB=0; FLI=0; BY=0.280117; FL=0; > FP=0; FZ=0; HX=0; KW=0; PH=0; RB=0; SC=0.415652; ST=0; TS=0; UL=0; ISC= > X-IBM-ISS-DetailInfo: BY=3.00004748; HX=3.00000237; KW=3.00000007; > PH=3.00000004; SC=3.00000129; SDB=6.00639970; UDB=6.00287883; UTC=2016-01-03 > 16:25:07 > X-TM-AS-MML: disable > > Hi Paul, Ingo, > > It seems to me that smp_read_barrier_depends (which resolves to > ___raw_smp_check_barrier_asm) is overdoing it, unless that particular part it is > written for is a disaster in terms of cache coherency. > > So far this is the only architecture that i know of (baring Alpha) which employs > non-empty read_barrier_depends(). I am wondering if this is really needed or > those who did the arch port got overly enthusiastic. If it is the former then > you may include another example of crazy architecture in your book. :) Hello, Petko, I must defer to the architecture maintainers. That said, there was a time when the blackfin maintainer were trying to make an SMP system without cache coherence, that is, by simply wiring two UP-only blackfin CPUs into a single system. They were using cache-flush tricks to make things more-or-less work. And their ___raw_smp_check_barrier_asm() does look to be flushing caches, so maybe that is what is happening here. And the comment header for read_barrier_depends() seems to support this view. Adding the blackfin folks and the usual lists on CC. Might get us something better than my half-remembered hearsay and possible misinterpretations of header comments. ;-) That said, cache-incoherent systems might well be a good addition to the book. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
