Date: Tue, 23 Jul 2024 09:41:17 +0200 From: Michal Meloun <meloun.michal@gmail.com> To: Konstantin Belousov <kib@freebsd.org> Cc: Mark Millard <marklmi@yahoo.com>, mmel@freebsd.org, FreeBSD Current <freebsd-current@freebsd.org>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: armv7-on-aarch64 stuck at urdlck Message-ID: <00b3e6be-648d-44c0-b95b-a31229fb5fde@gmail.com> In-Reply-To: <Zp7iN0PmCsdezNOP@kib.kiev.ua> References: <724db42b-5550-4381-8277-2971e6b3e8f1@freebsd.org> <B5E2275D-21F0-43C8-AF06-A45DB7448D66@yahoo.com> <86185657-e521-466b-89e2-f291aaac10a6@freebsd.org> <0EF18174-8735-46A4-BD71-FFA3472B319F@yahoo.com> <a1b978fe-ff54-4112-860c-b09500d89d0b@freebsd.org> <C0B42CBB-8F12-4597-A04B-26F2107E176E@yahoo.com> <33251aa3-681f-4d17-afe9-953490afeaf0@gmail.com> <Zp7iN0PmCsdezNOP@kib.kiev.ua>
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On 23.07.2024 0:50, Konstantin Belousov wrote: > On Mon, Jul 22, 2024 at 09:36:00PM +0200, Michal Meloun wrote: >> IMHO, -O2 shouldn't be able to modify function arguments for public >> functions, so <guessing> this memory corruption fits perfectly with the >> observed behavior</guessing>. >> >> But , out of curiosity, a quick look at _thr_rwlock_tryrdlock() in >> thr_umtx.h:208 makes me wonder: How is the "state" variable inside the loop >> guaranteed to be updated? IMHO nothing inside the loop emits a global memory >> modification attribute, so the compiler is free to move the assignment to a >> "state" variable outside the loop. >> > > I think that you are formally right, because there is only the _acq > atomic in the loop body, an evil compiler is allowed to move all loads > before the start of the loop iteration. > > But, since e.g. on arm32 atomic_cmpset_acq implementation contains dmb() > which provides the full barrier both for compiler memory accesses, and > for hw, it is not the case (for arm32). Thank you for the explanation. I didn't check the implementation of atomic_cmpset_acq and completely forgot that it has dmb() inside. My memory really needs a better refresh....
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