Date: Sun, 22 Feb 2015 01:12:39 +0000 From: "kmacy (Kip Macy)" <phabric-noreply@FreeBSD.org> To: freebsd-arm@freebsd.org Subject: [Differential] [Commented On] D1833: Add memory barriers to buf_ring Message-ID: <31c66607cc2e8a4ce997fcc05e223550@localhost.localdomain> In-Reply-To: <differential-rev-PHID-DREV-ntnotl4wj5wcj2eoukly-req@FreeBSD.org> References: <differential-rev-PHID-DREV-ntnotl4wj5wcj2eoukly-req@FreeBSD.org>
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kmacy added a comment. >>! In D1833#26, @meloun-miracle-cz wrote: > > Even I’m not able to evaluate all aspect, I see some serious defects here - at least from ARM point of view. > The buf_ring_enqueue() guarantees proper write ordering (and visibility): > - Store with acquire to br->br_prod_head > - Normal store to br_ring[] > - Store with release to br_prod_tail. > Unfortunately buf_ring_dequeue_sc() have not defined any read ordering and code can see > updated br_prod_tail and stale br_ring[]. (imho, this is true for amd64/i386 too). > Unlike of Semihalf guys, I see little different solution for race in buf_ring_dequeue_sc() read logic. > The br_ring[] must be read after br_prod_tail, but read order of br_cons_head and br_prod_tail is not important. > So (line 183) > > > buf = atomic_load_rel_32(&br->br_ring[cons_head]); > > > looks more correct to me. The update to prod_tail happens _after_ the store to br_ring and is done with an atomic_store_rel_int subsequently in dequeue_sc the read of prod_tail will be done with an atomic_load_acq_int before loading br_prod. Since there is a memory barrier after the update in enqueue and one before the read where is the problem? > > On ARM, all stores to variable referenced by atomic_cmpset() must be done by atomic_store (or by other > atomic_* functions), normal store doesn’t clear exclusive monitor. > Thus, for ARM we **!!MUST!!** use atomic_store() for each store to variable referenced by atomic_cmpset() ! Great, but the only variable that is updated with atomic_cmpset is prod_head to atomically acquire the right to store in to that index in br_ring. Please clarify what the actual problem is. This change fixes the only real issue that I can glean from your comment. REVISION DETAIL https://reviews.freebsd.org/D1833 To: zbb, kmacy, rpaulo, imp Cc: meloun-miracle-cz, onwahe-gmail-com, andrew, ian, adrian, freebsd-arm
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