Date: Tue, 6 Dec 2016 00:54:14 +0000 From: "oleg (Oleg Bulyzhin)" <phabric-noreply@FreeBSD.org> To: freebsd-net@freebsd.org Subject: [Differential] D8637: buf_ring.h: fix memory order issues. Message-ID: <24dce39a56da9a22a2bfcb1677195ff8@localhost.localdomain> In-Reply-To: <differential-rev-PHID-DREV-7phoe4ltzb7cywaexwdr-req@FreeBSD.org> References: <differential-rev-PHID-DREV-7phoe4ltzb7cywaexwdr-req@FreeBSD.org>
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oleg added a comment. In https://reviews.freebsd.org/D8637#180625, @alc wrote: > Have you looked at https://reviews.freebsd.org/D1945, in particular, the most recent postings by sbahra_repnop.org? It's not clear to me that these changes will address the problem described in sbahra_repnop.org's postings. That said, your proposed changes do correct the most obvious remaining issues with the use of acquires and releases in this code. Yes, i've looked at https://reviews.freebsd.org/D1945 but somehow i've missed sbahra_repnop.org's comments. Since i was trying to fix different issue (https://lists.freebsd.org/pipermail/freebsd-net/2013-December/037265.html) and remove my own rmb() hack the problem described by sbahra_repnop.org is still here. I'll post updated diff tomorrow. INLINE COMMENTS > alc wrote in buf_ring.h:98 > You may need to use a load acquire on br_prod_tail here to establish an unbroken synchronizes-with chain between the thread that enqueues an item X and the thread that later dequeues it if there are other concurrent enqueues. AFAIK 'synchronizes-with chains' are about RMW operations in between load_acq/store_rel sequence. Can you explain why load_acq is necessary here? I think producer should not care about visibility of br->br_ring[prod_head] stores of other producers. Correct order of our own stores guaranteed by store_rel(&br->br_prod_tail) (and load_acq() in consumer). REVISION DETAIL https://reviews.freebsd.org/D8637 EMAIL PREFERENCES https://reviews.freebsd.org/settings/panel/emailpreferences/ To: oleg, kmacy, kib, alc Cc: emaste, freebsd-net-listhelp
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