Date: Thu, 09 Mar 2017 14:52:09 +0100 From: Alexandre Martins <alexandre.martins@stormshield.eu> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-current <freebsd-current@freebsd.org> Subject: Re: smp_rendezvous_action: Are atomics correctly used ? Message-ID: <3034263.GEZH9i4V44@pc-alex> In-Reply-To: <20170309130754.GX30979@kib.kiev.ua> References: <2092905.6A8RAGlt18@pc-alex> <4355496.HzrDjoMEL6@pc-alex> <20170309130754.GX30979@kib.kiev.ua>
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--nextPart12410435.8xKqY3EYM9 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" Le jeudi 9 mars 2017, 15:07:54 Konstantin Belousov a =E9crit : > On Thu, Mar 09, 2017 at 10:59:27AM +0100, Alexandre Martins wrote: > > I have the save question for the cpu_ipi_pending here: > >=20 > > https://svnweb.freebsd.org/base/head/sys/x86/x86/mp_x86.c?view=3Dan= notate#l1 > > 080>=20 > > Le jeudi 9 mars 2017, 10:43:14 Alexandre Martins a ?crit : > > > Hello, > > >=20 > > > I'm curently reading the code of the function smp_rendezvous_acti= on, in > > > kern/subr_smp.c file. In that function, i see that the variable > > > smp_rv_waiters is read in some while() loop in a non-atomic way. > > >=20 > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=3Da= nnotate#l > > > 412 > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=3Da= nnotate#l > > > 458 > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=3Da= nnotate#l > > > 472 > > >=20 > > > I suspect one of my freeze to be due by that. >=20 > You should provide either evidence or, at least, some reasoning suppo= rting > your claims. I curently have a software watchdog that triger and does a coredump. In= the=20 coredumps, I always see a CPU trying to write-lock a "rm lock". Every t= ime,=20 that CPU is spinning into the smp_rendezvous_action, in the first while= loop)=20 while the others are into the idle threads. The fact is that freeze is not clear and I start to search "exotic" cau= ses to=20 explain it. >=20 > > > Should this function be patched to use > > > "atomic_load_acq_int(&smp_rv_waiters[])" ? >=20 > There too. >=20 > As a side note, any read or write of the naturally aligned integer > types with size less or equal than the machine word, on all supported= > architectures, are atomic. The meaning of the word atomic there is > that when reading, you always get a complete value that was written b= y > a writer into this location, not some out of thin air value. Similar= ly, > when writing, you are guaranteed that any observer of the write will = see > the value you have wrote. >=20 > The guarantees above hold both for C-level code and for the assembler= > accesses. >=20 > atomic_load_acq() provides additional guarantees which do not affect = the > value read from the variable itself, but establish the ordering on th= e > visibility of the related operations. OK, I got it. Thank you ! =2D-=20 Alexandre Martins STORMSHIELD --nextPart12410435.8xKqY3EYM9 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCCOUw ggSGMIICbqADAgECAgUA28zw7TANBgkqhkiG9w0BAQsFADBIMQswCQYDVQQGEwJGUjEUMBIGA1UE CgwLU1RPUk1TSElFTEQxIzAhBgNVBAMMGlN0b3Jtc2hpZWxkIFJvb3QgQXV0aG9yaXR5MB4XDTE0 MDkwNDE1MDcxMFoXDTI0MDkwMTE1MDcxMFowSTELMAkGA1UEBhMCRlIxFDASBgNVBAoMC1NUT1JN U0hJRUxEMSQwIgYDVQQDDBtTdG9ybXNoaWVsZCBVc2VycyBBdXRob3JpdHkwggEiMA0GCSqGSIb3 DQEBAQUAA4IBDwAwggEKAoIBAQDChwWgC/6VWKL7jgWI3eA2sVvRdOwuHcXsRAAXVWdlMC0ygg7u 45E78GhAnpdl8QbIu7x/Q2zOq6KttspwDEIjkoMLTZngLLlGjYJZPfuSoC6hl9R7vRd5f8Fhu3v0 xQ/7vzKYz4C836IGCrk31gmrPO0H0fxkyxCMfhoTTzue3oXW1IsmQwCrOPOu2Y82QANDhbifWLjI WJetnj58YRKR82KBs3Flbqxtp0mi9+IswMvCCRSoT+ORB73Cl6URt7Qm7BcD+qnkJ9uwlUC94dIl T2hX4ybY/w/ssA17Ew418fgyRCWQXzgjZgZ/XUcw2WP9dIggA7Pg+c/xeROJH1zvAgMBAAGjdjB0 MB0GA1UdDgQWBBShbYRsooCFBXx8dXWANMETW5fXgTAfBgNVHSMEGDAWgBS4Qqn6Z0Twf9NhjOyl x1CutL3sozAPBgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQEAwIBBjARBglghkgBhvhCAQEEBAMC AQYwDQYJKoZIhvcNAQELBQADggIBAE6C9zkt2J6dPm2KLbzRS6rBIYZNFi0X59g3ekQ2Sc4UWsq+ B3L86j5xnQSRnIM/DKV1+Q2UHbU/qsh4cto2fwTV6V+aJ07Vu/bJE1rAN4AI4V26ytf7VoBcBjVZ Jq8pHOMp/G2eQH7F1xqzml68DpKku66aUalkcC9IM82m7AW3YAyvDoYEAchv4qyL8qhVLLp6LNru 8ZOhMELhZLWl4ulw/SFDMhcBS6I4wC6icj71MLGSrr61vMktMdwQ+CGFQ5z5JbaxM61VgzKay8+g lw+xTbpnitrDfhkzHs2fdwOOur3vtNnNsrdBWiYPseJ2k4VGD7ov5kITQZckmZyF/V+Ir//agJQG VuwhDZCXgXOvrje+FLYp7tQ9pgSvLbluh1A+ywfyHnFI4n6tZy9SD3MIDgWR4KwFLM1Qmt3NQb32 tkq9Vm0jUcQXFfbnWKLA9krw3m8NmCqhL5PzpfOegYOc0QJWfMQamxeWxXMLk6uKisS//+VqfpCa 5Jx53t+9DmoN1+ob4jOprPaX6tfBBr5djah2yzPGjHEB52VgWXxIF9lCM2z7Qw+zFb2PIdNeSjIk NEFg/1orKAAa5gQXAQynN2J7E+aLf2XLhHcS0v+9yoisPEw9+Tb5F1uQh+gzYD5JUUYcYWncnX8g P8k6X+F5mQ/81IoNL/IejxJgy/LoMIIEVzCCAz+gAwIBAgIFAIUoy7swDQYJKoZIhvcNAQELBQAw STELMAkGA1UEBhMCRlIxFDASBgNVBAoMC1NUT1JNU0hJRUxEMSQwIgYDVQQDDBtTdG9ybXNoaWVs ZCBVc2VycyBBdXRob3JpdHkwHhcNMTYwOTAxMTUxMTA4WhcNMTcwOTAxMTUxMTA4WjBwMQswCQYD VQQGEwJGUjEUMBIGA1UECgwLU1RPUk1TSElFTEQxGjAYBgNVBAMMEUFsZXhhbmRyZSBNQVJUSU5T MS8wLQYJKoZIhvcNAQkBFiBhbGV4YW5kcmUubWFydGluc0BzdG9ybXNoaWVsZC5ldTCCASIwDQYJ KoZIhvcNAQEBBQADggEPADCCAQoCggEBAMN+CnvE13jKEwJ+OyMzwBpC02dY+LpD5luJwnJTVnV2 9aUjEMI+xGFMMHd9kSIVInbk4WDe1ELOKerg0dzgnkRiOHECSGum1UhcZABxQgY2cmSffNQ6JVro 52UaBlt3aTOk3imYJCHUIGgOWMvOtRc8BxyBHdi15FZPj/F9I+AKufRFsBXUakplFIAPEwy3m2eR a/eCMLqGJUyK7YmsAlEnYn2mA38zIoqtKvL6KPHtrV8fw1SRLQ13+j9nu1LlCaqhmLtILFxhV0/9 uDTvx5cKtZ8Xj1nPM6NUUrso9qlXwm4On6Y34pVTtnYGMQRuljil3Hiz84RJjPDJYRGwbgkCAwEA AaOCAR0wggEZMB0GA1UdDgQWBBTmRLIwSfhNwbdfV13xt0G0JHYjPDAfBgNVHSMEGDAWgBShbYRs ooCFBXx8dXWANMETW5fXgTAJBgNVHRMEAjAAMA4GA1UdDwEB/wQEAwID6DARBglghkgBhvhCAQEE BAMCBLAwHQYDVR0lBBYwFAYIKwYBBQUHAwQGCCsGAQUFBwMCMEoGA1UdHwRDMEEwP6A9oDuGOWh0 dHBzOi8vcGtpLm5ldGFzcS5jb20vYXV0aC9jZXJ0aWZpY2F0ZXJldm9jYXRpb25saXN0LmNybDAR BgNVHSAECjAIMAYGBFUdIAAwKwYDVR0RBCQwIoEgYWxleGFuZHJlLm1hcnRpbnNAc3Rvcm1zaGll bGQuZXUwDQYJKoZIhvcNAQELBQADggEBALT9NWiAaE6nDev34vShhsyb9lWBOQfCnAMyKwtFy/cU uIoHsxyOanIIQHz0ZtB76GCHDo7RStMyp6RYIefIsxABLhSr4hHapJka9g/X/nxexyr0xyT3IpYQ dmyMSHRT18Z/ZaBlQdyfnS2PYkPHJAHl4iqB4SnQlh3rwFdKTJMgCz413cDxQHytgRPGTiXOhyV7 aS3ANJFha6ZHA8HU9sTslY8ZXSUu94iD3t2kcF3gBb432UKALwryKqnrwzFX68pFpqO5QAjEHaF6 6p1agMb2b3HlQGZrME5wSO6rsZJPYvJEyvrwHxCxjSTkOdPw6GriWGTMrVMU0fVrfptMS1gxggIT MIICDwIBATBSMEkxCzAJBgNVBAYTAkZSMRQwEgYDVQQKDAtTVE9STVNISUVMRDEkMCIGA1UEAwwb U3Rvcm1zaGllbGQgVXNlcnMgQXV0aG9yaXR5AgUAhSjLuzANBglghkgBZQMEAgEFAKCBkzAYBgkq hkiG9w0BCQMxCwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0xNzAzMDkxMzUyMDlaMCgGCSqG SIb3DQEJDzEbMBkwCwYJYIZIAWUDBAECMAoGCCqGSIb3DQMHMC8GCSqGSIb3DQEJBDEiBCAfZAsZ nWM8Cs2okcqQdQQolSaGh2vRXhdaz/RKEM3FJTANBgkqhkiG9w0BAQEFAASCAQBk2g/0huvjVtrj pcNgg8FKY5CwnZi0LjJpQLxAd/Y2Nf5lCuA/6bUiCli64391TlrZaSb35nt2Z/T2oumFkxcW56SX LPjOXFFcOr6kMRvYsGoe6UCnqvqqaT5YoiilhCtJSsntooDx7EjBUNheXSW06ewdWGeKM8STQnL9 kg+KVRJi7BvI5z8h6tkF1DoLq2M7QeSdhbOYqVQfxbucb4byouH/TsowTMFHjH1UAPFjukMh1YAA 5PffWoB7MdTUoU50UKJN9cdPP4Ug8q2X0IJ3Gd3jh3XYlJ1VSGsnO/wkut+jDXuYEEG5bVGj3oh5 cJVlRsUGNdfDji/q0GdU2h6vAAAAAAAA --nextPart12410435.8xKqY3EYM9--
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