Date: Tue, 23 Jul 2024 01:50:31 +0300 From: Konstantin Belousov <kib@freebsd.org> To: Michal Meloun <meloun.michal@gmail.com> 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: <Zp7iN0PmCsdezNOP@kib.kiev.ua> In-Reply-To: <33251aa3-681f-4d17-afe9-953490afeaf0@gmail.com> 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>
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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).home | help
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