Date: Mon, 9 Mar 2020 01:56:17 +0100 From: Mateusz Guzik <mjguzik@gmail.com> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r358733 - head/sys/sys Message-ID: <CAGudoHE4hTN5niCAsA4tfggywkEnGxiaoPz=rTX3AHA=fgZfkQ@mail.gmail.com> In-Reply-To: <202003080022.0280MX9j055805@repo.freebsd.org> References: <202003080022.0280MX9j055805@repo.freebsd.org>
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On 3/8/20, Mateusz Guzik <mjg@freebsd.org> wrote: > Author: mjg > Date: Sun Mar 8 00:22:32 2020 > New Revision: 358733 > URL: https://svnweb.freebsd.org/changeset/base/358733 > > Log: > seqc: tidy up > > - avoid hand-rolled read > - match begin/end in terms of fence style > There were off lists questions about this so let me clarify. The first bit is a cosmetic change, but the second one is not. > Modified: > head/sys/sys/seqc.h > > Modified: head/sys/sys/seqc.h > ============================================================================== > --- head/sys/sys/seqc.h Sat Mar 7 15:37:23 2020 (r358732) > +++ head/sys/sys/seqc.h Sun Mar 8 00:22:32 2020 (r358733) > @@ -66,7 +66,8 @@ static __inline void > seqc_write_end(seqc_t *seqcp) > { > > - atomic_store_rel_int(seqcp, *seqcp + 1); > + atomic_thread_fence_rel(); > + *seqcp += 1; > MPASS(!seqc_in_modify(*seqcp)); > critical_exit(); > } For correct operation the counter has to be modified *before* any work is done and *after* it is completed. Consider a trivial case: seqc++; foo[0] = 0; foo[1] = 1; seqc++; There are 2 ways in which this can be mucked with: - the compiler can be looking at reordering or reworking the increment (e.g., to collapse it to just += 2 instead). a compiler barrier prevents that. - even with generated machine code which increments in correct places the cpu can be looking at reordering the operation (which is heavily dependent on architecture), in particular it could end up issuing seqc++; foo[1] = 1; which would defeat the point. This is where release fences come in. With the patched code there is: seqc++; atomic_thread_fence_rel(); /* make sure seqc++ is issued before foo starts getting modified */ foo[0] = 0; foo[1] = 1; atomic_thread_fence_rel(); /* make sure modifications to foo don't leak past this spot */ seqc++; In comparison, the previous code was: seqc++; atomic_thread_fence_rel(); /* make sure seqc++ is issued before foo starts getting modified */ foo[0] = 0; foo[1] = 1; atomic_store_rel_int(seqcp, *seqcp + 1); /* make sure modifications to too don't leak past this spot and update seqc immediately */ There are 2 differences here -- one is an improvement and second one does not matter: the win is: the previous code forces the compiler to compute an incremented value and then store it. On amd64 this translates to the following (with rdx holding the address of the counter): mov (%rdx),%eax /* load the value */ add $0x1,%eax /* add 1 */ mov %eax,(%rdx) /* store it */ On patched kernel this is: addl $0x1,(%rdx) which is clearly much nicer. the not a problem: the code does not issue the release fence after incrementing the counter the second time, meaning that in principle it can happen arbitrarily late, possibly disturbing code which waits for the counter to become even. This is not a problem because in whoever modifies it is supposed to have a lock and release it shortly after, which also posts the release fence and consequently makes sure the counter update is visible. Also note these routines come with preemption disablement around the modification -- if the thread gets preempted as a result of critical_exit from seqc_write_end, it will also publish the updated counter. If the updater never does anything which flushes the store buffer then in the absolute worst case they will get preempted, at which point once more we are covered. Note that code which wants more deterministic behavior will want to seqc_read_any, test the counter once and if it is caught uneven resort to locking. > @@ -84,7 +85,7 @@ seqc_read(const seqc_t *seqcp) > seqc_t ret; > > for (;;) { > - ret = atomic_load_acq_int(__DECONST(seqc_t *, seqcp)); > + ret = seqc_read_any(seqcp); > if (__predict_false(seqc_in_modify(ret))) { > cpu_spinwait(); > continue; > _______________________________________________ > svn-src-all@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/svn-src-all > To unsubscribe, send any mail to "svn-src-all-unsubscribe@freebsd.org" > -- Mateusz Guzik <mjguzik gmail.com>
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