From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mikael Pettersson Subject: Re: bit fields && data tearing Date: Thu, 4 Sep 2014 10:57:40 +0200 Message-ID: <21512.10628.412205.873477@gargle.gargle.HOWL> References: <20140712181328.GA8738@redhat.com> <54079B70.4050200@hurleysoftware.com> <1409785893.30640.118.camel@pasglop> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: In-Reply-To: <1409785893.30640.118.camel@pasglop> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+glppe-linuxppc-embedded-2=m.gmane.org@lists.ozlabs.org Sender: "Linuxppc-dev" To: Benjamin Herrenschmidt Cc: Jakub Jelinek , linux-arch@vger.kernel.org, Tony Luck , linux-ia64@vger.kernel.org, Peter Hurley , Oleg Nesterov , linux-kernel@vger.kernel.org, Paul Mackerras , "Paul E. McKenney" , linuxppc-dev@lists.ozlabs.org, Miroslav Franc , Richard Henderson List-Id: linux-arch.vger.kernel.org QmVuamFtaW4gSGVycmVuc2NobWlkdCB3cml0ZXM6CiA+IE9uIFdlZCwgMjAxNC0wOS0wMyBhdCAx ODo1MSAtMDQwMCwgUGV0ZXIgSHVybGV5IHdyb3RlOgogPiAKID4gPiBBcG9sb2dpZXMgZm9yIGhp amFja2luZyB0aGlzIHRocmVhZCBidXQgSSBuZWVkIHRvIGV4dGVuZCB0aGlzIGRpc2N1c3Npb24K ID4gPiBzb21ld2hhdCByZWdhcmRpbmcgd2hhdCBhIGNvbXBpbGVyIG1pZ2h0IGRvIHdpdGggYWRq YWNlbnQgZmllbGRzIGluIGEgc3RydWN0dXJlLgogPiA+IAogPiA+IFRoZSB0dHkgc3Vic3lzdGVt IGRlZmluZXMgYSBsYXJnZSBhZ2dyZWdhdGUgc3RydWN0dXJlLCBzdHJ1Y3QgdHR5X3N0cnVjdC4K ID4gPiBJbXBvcnRhbnRseSwgc2V2ZXJhbCBkaWZmZXJlbnQgbG9ja3MgYXBwbHkgdG8gZGlmZmVy ZW50IGZpZWxkcyB3aXRoaW4gdGhhdAogPiA+IHN0cnVjdHVyZTsgaWUuLCBhIHNwZWNpZmljIHNw aW5sb2NrIHdpbGwgYmUgY2xhaW1lZCBiZWZvcmUgdXBkYXRpbmcgb3IgYWNjZXNzaW5nCiA+ID4g Y2VydGFpbiBmaWVsZHMgd2hpbGUgYSBkaWZmZXJlbnQgc3BpbmxvY2sgd2lsbCBiZSBjbGFpbWVk IGJlZm9yZSB1cGRhdGluZyBvcgogPiA+IGFjY2Vzc2luZyBjZXJ0YWluIF9hZGphY2VudF8gZmll bGRzLgogPiA+IAogPiA+IFdoYXQgaXMgbmVjZXNzYXJ5IGFuZCBzdWZmaWNpZW50IHRvIHByZXZl bnQgYWNjaWRlbnRhbCBmYWxzZS1zaGFyaW5nPwogPiA+IFRoZSBwYXRjaCBiZWxvdyB3YXMgZmxh Z2dlZCBhcyBpbnN1ZmZpY2llbnQgb24gaWE2NCwgYW5kIHBvc3NpYmx5IEFSTS4KID4gCiA+IFdl IGV4cGVjdCBuYXRpdmUgYWxpZ25lZCBzY2FsYXIgdHlwZXMgdG8gYmUgYWNjZXNzZWQgYXRvbWlj YWxseSAodGhlCiA+IHJlYWQvbW9kaWZ5L3dyaXRlIG9mIGEgbGFyZ2VyIHF1YW50aXR5IHRoYXQg Z2NjIGRvZXMgb24gc29tZSBiaXRmaWVsZAogPiBjYXNlcyBoYXMgYmVlbiBmbGFnZ2VkIGFzIGEg Z2NjIGJ1ZywgYnV0IHNob3VsZG4ndCBoYXBwZW4gb24gbm9ybWFsCiA+IHNjYWxhciB0eXBlcyku CiA+IAogPiBJIGFtIG5vdCAxMDAlIGNlcnRhaW4gb2YgImJvb2wiIGhlcmUsIEkgYXNzdW1lIGl0 J3MgdHJlYXRlZCBhcyBhIG5vcm1hbAogPiBzY2FsYXIgYW5kIHRodXMgYXRvbWljIGJ1dCBpZiB1 bnN1cmUsIHlvdSBjYW4gYWx3YXlzIHVzZSBpbnQuCgpQbGVhc2UgdXNlIGFuIGFsaWduZWQgaW50 IG9yIGxvbmcuICBTb21lIG1hY2hpbmVzIGNhbm5vdCBkbyBhdG9taWMKYWNjZXNzZXMgb24gc3Vi LWludC9sb25nIHF1YW50aXRpZXMsIHNvICdib29sJyBtYXkgY2F1c2UgdW5leHBlY3RlZApybXcg Y3ljbGVzIG9uIGFkamFjZW50IGZpZWxkcy4KCi9NaWthZWwKCiA+IAogPiBBbm90aGVyIG9wdGlv biBpcyB0byB1c2UgdGhlIGF0b21pYyBiaXRvcHMgYW5kIG1ha2UgdGhlc2UgYml0cyBpbiBhCiA+ IGJpdG1hc2sgYnV0IHRoYXQgaXMgcHJvYmFibHkgdW5uZWNlc3NhcnkgaWYgeW91IGhhdmUgbG9j a3MgYWxyZWFkeS4KID4gCiA+IENoZWVycywKID4gQmVuLgogPiAKID4gCiA+ID4gUmVnYXJkcywK ID4gPiBQZXRlciBIdXJsZXkKID4gPiAKID4gPiAtLS0gPiUgLS0tCiA+ID4gU3ViamVjdDogW1BB VENIIDIxLzI2XSB0dHk6IENvbnZlcnQgdHR5X3N0cnVjdCBiaXRmaWVsZCB0byBib29scwogPiA+ IAogPiA+IFRoZSBzdG9wcGVkLCBod19zdG9wcGVkLCBmbG93X3N0b3BwZWQgYW5kIHBhY2tldCBi aXRzIGFyZSBzbXAtdW5zYWZlCiA+ID4gYW5kIGludGVycnVwdC11bnNhZmUuIEZvciBleGFtcGxl LAogPiA+IAogPiA+IENQVSAwICAgICAgICAgICAgICAgICAgICAgICAgIHwgQ1BVIDEKID4gPiAg ICAgICAgICAgICAgICAgICAgICAgICAgICAgICB8CiA+ID4gdHR5LT5mbG93X3N0b3BwZWQgPSAx ICAgICAgICAgfCB0dHktPmh3X3N0b3BwZWQgPSAwCiA+ID4gCiA+ID4gT25lIG9mIHRoZXNlIHVw ZGF0ZXMgd2lsbCBiZSBjb3JydXB0ZWQsIGFzIHRoZSBiaXR3aXNlIG9wZXJhdGlvbgogPiA+IG9u IHRoZSBiaXRmaWVsZCBpcyBub24tYXRvbWljLgogPiA+IAogPiA+IEVuc3VyZSBlYWNoIGZsYWcg aGFzIGEgc2VwYXJhdGUgbWVtb3J5IGxvY2F0aW9uLCBzbyBjb25jdXJyZW50CiA+ID4gdXBkYXRl cyBkbyBub3QgY29ycnVwdCBvcnRob2dvbmFsIHN0YXRlcy4KID4gPiAKID4gPiBTaWduZWQtb2Zm LWJ5OiBQZXRlciBIdXJsZXkgPHBldGVyQGh1cmxleXNvZnR3YXJlLmNvbT4KID4gPiAtLS0KID4g PiAgaW5jbHVkZS9saW51eC90dHkuaCB8IDUgKysrKy0KID4gPiAgMSBmaWxlIGNoYW5nZWQsIDQg aW5zZXJ0aW9ucygrKSwgMSBkZWxldGlvbigtKQogPiA+IAogPiA+IGRpZmYgLS1naXQgYS9pbmNs dWRlL2xpbnV4L3R0eS5oIGIvaW5jbHVkZS9saW51eC90dHkuaAogPiA+IGluZGV4IDFjMzMxNmEu LjdjZjYxY2IgMTAwNjQ0CiA+ID4gLS0tIGEvaW5jbHVkZS9saW51eC90dHkuaAogPiA+ICsrKyBi L2luY2x1ZGUvbGludXgvdHR5LmgKID4gPiBAQCAtMjYxLDcgKzI2MSwxMCBAQCBzdHJ1Y3QgdHR5 X3N0cnVjdCB7CiA+ID4gIAl1bnNpZ25lZCBsb25nIGZsYWdzOwogPiA+ICAJaW50IGNvdW50Owog PiA+ICAJc3RydWN0IHdpbnNpemUgd2luc2l6ZTsJCS8qIHdpbnNpemVfbXV0ZXggKi8KID4gPiAt CXVuc2lnbmVkIGNoYXIgc3RvcHBlZDoxLCBod19zdG9wcGVkOjEsIGZsb3dfc3RvcHBlZDoxLCBw YWNrZXQ6MTsKID4gPiArCWJvb2wgc3RvcHBlZDsKID4gPiArCWJvb2wgaHdfc3RvcHBlZDsKID4g PiArCWJvb2wgZmxvd19zdG9wcGVkOwogPiA+ICsJYm9vbCBwYWNrZXQ7CiA+ID4gIAl1bnNpZ25l ZCBjaGFyIGN0cmxfc3RhdHVzOwkvKiBjdHJsX2xvY2sgKi8KID4gPiAgCXVuc2lnbmVkIGludCBy ZWNlaXZlX3Jvb207CS8qIEJ5dGVzIGZyZWUgZm9yIHF1ZXVlICovCiA+ID4gIAlpbnQgZmxvd19j aGFuZ2U7CiA+IAogPiAKID4gLS0KID4gVG8gdW5zdWJzY3JpYmUgZnJvbSB0aGlzIGxpc3Q6IHNl bmQgdGhlIGxpbmUgInVuc3Vic2NyaWJlIGxpbnV4LWtlcm5lbCIgaW4KID4gdGhlIGJvZHkgb2Yg YSBtZXNzYWdlIHRvIG1ham9yZG9tb0B2Z2VyLmtlcm5lbC5vcmcKID4gTW9yZSBtYWpvcmRvbW8g aW5mbyBhdCAgaHR0cDovL3ZnZXIua2VybmVsLm9yZy9tYWpvcmRvbW8taW5mby5odG1sCiA+IFBs ZWFzZSByZWFkIHRoZSBGQVEgYXQgIGh0dHA6Ly93d3cudHV4Lm9yZy9sa21sLwoKLS0gCl9fX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fCkxpbnV4cHBjLWRldiBt YWlsaW5nIGxpc3QKTGludXhwcGMtZGV2QGxpc3RzLm96bGFicy5vcmcKaHR0cHM6Ly9saXN0cy5v emxhYnMub3JnL2xpc3RpbmZvL2xpbnV4cHBjLWRldg== From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f52.google.com ([209.85.215.52]:33252 "EHLO mail-la0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754711AbaIDI5p (ORCPT ); Thu, 4 Sep 2014 04:57:45 -0400 From: Mikael Pettersson MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <21512.10628.412205.873477@gargle.gargle.HOWL> Date: Thu, 4 Sep 2014 10:57:40 +0200 Subject: Re: bit fields && data tearing In-Reply-To: <1409785893.30640.118.camel@pasglop> References: <20140712181328.GA8738@redhat.com> <54079B70.4050200@hurleysoftware.com> <1409785893.30640.118.camel@pasglop> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Benjamin Herrenschmidt Cc: Peter Hurley , Jakub Jelinek , "Paul E. McKenney" , Richard Henderson , Oleg Nesterov , Miroslav Franc , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Tony Luck , linux-ia64@vger.kernel.org Message-ID: <20140904085740.RSn6zCDySGIQxSyymt3zRtClGjGUGmAgaK5MfJraZgs@z> Benjamin Herrenschmidt writes: > On Wed, 2014-09-03 at 18:51 -0400, Peter Hurley wrote: > > > Apologies for hijacking this thread but I need to extend this discussion > > somewhat regarding what a compiler might do with adjacent fields in a structure. > > > > The tty subsystem defines a large aggregate structure, struct tty_struct. > > Importantly, several different locks apply to different fields within that > > structure; ie., a specific spinlock will be claimed before updating or accessing > > certain fields while a different spinlock will be claimed before updating or > > accessing certain _adjacent_ fields. > > > > What is necessary and sufficient to prevent accidental false-sharing? > > The patch below was flagged as insufficient on ia64, and possibly ARM. > > We expect native aligned scalar types to be accessed atomically (the > read/modify/write of a larger quantity that gcc does on some bitfield > cases has been flagged as a gcc bug, but shouldn't happen on normal > scalar types). > > I am not 100% certain of "bool" here, I assume it's treated as a normal > scalar and thus atomic but if unsure, you can always use int. Please use an aligned int or long. Some machines cannot do atomic accesses on sub-int/long quantities, so 'bool' may cause unexpected rmw cycles on adjacent fields. /Mikael > > Another option is to use the atomic bitops and make these bits in a > bitmask but that is probably unnecessary if you have locks already. > > Cheers, > Ben. > > > > Regards, > > Peter Hurley > > > > --- >% --- > > Subject: [PATCH 21/26] tty: Convert tty_struct bitfield to bools > > > > The stopped, hw_stopped, flow_stopped and packet bits are smp-unsafe > > and interrupt-unsafe. For example, > > > > CPU 0 | CPU 1 > > | > > tty->flow_stopped = 1 | tty->hw_stopped = 0 > > > > One of these updates will be corrupted, as the bitwise operation > > on the bitfield is non-atomic. > > > > Ensure each flag has a separate memory location, so concurrent > > updates do not corrupt orthogonal states. > > > > Signed-off-by: Peter Hurley > > --- > > include/linux/tty.h | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/tty.h b/include/linux/tty.h > > index 1c3316a..7cf61cb 100644 > > --- a/include/linux/tty.h > > +++ b/include/linux/tty.h > > @@ -261,7 +261,10 @@ struct tty_struct { > > unsigned long flags; > > int count; > > struct winsize winsize; /* winsize_mutex */ > > - unsigned char stopped:1, hw_stopped:1, flow_stopped:1, packet:1; > > + bool stopped; > > + bool hw_stopped; > > + bool flow_stopped; > > + bool packet; > > unsigned char ctrl_status; /* ctrl_lock */ > > unsigned int receive_room; /* Bytes free for queue */ > > int flow_change; > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ --