Author: kib Date: Mon May 21 18:41:16 2018 New Revision: 333990 URL: https://svnweb.freebsd.org/changeset/base/333990
Log: Add missed barrier for pm_gen/pm_active interaction. When we issue shootdown IPIs, we first assign zero to pm_gens to indicate the need to flush on the next context switch in case our IPI misses the context, next we read pm_active. On context switch we set our bit in pm_active, then we read pm_gen. It is crucial that both threads see the memory in the program order, otherwise invalidation thread might read pm_active bit as zero and the context switching thread might read pm_gen as zero. IA32 allows CPU for both reads to see zero. We must use the barriers between write and read. The pm_active bit set is already locked, so only the invalidation functions need it. I never saw it in real life, or at least I do not have a good reproduction case. I found this during code inspection when hunting for the Xen TLB issue reported by cperciva. Reviewed by: alc, markj Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D15506 Modified: head/sys/amd64/amd64/pmap.c Modified: head/sys/amd64/amd64/pmap.c ============================================================================== --- head/sys/amd64/amd64/pmap.c Mon May 21 17:33:52 2018 (r333989) +++ head/sys/amd64/amd64/pmap.c Mon May 21 18:41:16 2018 (r333990) @@ -1721,6 +1721,18 @@ pmap_invalidate_page(pmap_t pmap, vm_offset_t va) if (cpuid != i) pmap->pm_pcids[i].pm_gen = 0; } + + /* + * The fence is between stores to pm_gen and the read of + * the pm_active mask. We need to ensure that it is + * impossible for us to miss the bit update in pm_active + * and simultaneously observe a non-zero pm_gen in + * pmap_activate_sw(), otherwise TLB update is missed. + * Without the fence, IA32 allows such an outcome. + * Note that pm_active is updated by a locked operation, + * which provides the reciprocal fence. + */ + atomic_thread_fence_seq_cst(); } mask = &pmap->pm_active; } @@ -1792,6 +1804,8 @@ pmap_invalidate_range(pmap_t pmap, vm_offset_t sva, vm if (cpuid != i) pmap->pm_pcids[i].pm_gen = 0; } + /* See comment int pmap_invalidate_page(). */ + atomic_thread_fence_seq_cst(); } mask = &pmap->pm_active; } @@ -1863,6 +1877,8 @@ pmap_invalidate_all(pmap_t pmap) if (cpuid != i) pmap->pm_pcids[i].pm_gen = 0; } + /* See comment int pmap_invalidate_page(). */ + atomic_thread_fence_seq_cst(); } mask = &pmap->pm_active; } _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
