MFC r305325: MFV r303078:

MFC r305325: MFV r303078:
7086 ztest attempts dva_get_dsize_sync on an embedded blockpointer

illumos/illumos-gate@926549256b71acd595f69b236779ff6b78fa08ef
https://github.com/illumos/illumos-gate/commit/926549256b71acd595f69b236779ff6b7
8fa08ef

https://www.illumos.org/issues/7086
  In dbuf_dirty(), we need to grab the dn_struct_rwlock before looking at the
  db_blkptr, to prevent it from being changed by syncing context.
  Otherwise we may see that ztest got a segfault from this stack:
  libzpool.so.1`dva_get_dsize_sync+0x98(872f000, b32b240, fed7811b, 0, b4cda20,
0)
  libzpool.so.1`bp_get_dsize+0x60(872f000, b32b240, 0, 97cb780, 9d4c1a8, 0)
  libzpool.so.1`dbuf_dirty+0x9b3(ce0a100, 97cb780, 9, fecd2530)
  libzpool.so.1`dmu_buf_will_dirty+0xc3(ce0a100, 97cb780, ea293d6c, 1)
  libzpool.so.1`zap_lockdir+0x1a0(8aaa3c0, 1, 0, 97cb780, 1, 1)
  libzpool.so.1`zap_remove_norm+0x30(8aaa3c0, 1, 0, 8728b10, 0, 97cb780)
  libzpool.so.1`zap_remove+0x29(8aaa3c0, 1, 0, 8728b10, 97cb780, a)
  ztest_replay_remove+0x225(ea294588, 8728ae8, 0, 38010000, 0, 0)
  ztest_remove+0x9f(ea294588, ea293f50, 4, 3)
  ztest_object_init+0x78(ea294588, ea293f50, 4e0, 1)
  ztest_dmu_object_alloc_free+0x71(ea294588, 13)
  ztest_dmu_objset_create_destroy+0x224(80cef08, 13, 0, 805d36c, 9017ad44, 0)
  ztest_execute+0x89(a, 807c720, 13, 0)
  ztest_thread+0xea(13, 0, 0, 0)
  libc.so.1`_thrp_setup+0x88(f0983240)
  libc.so.1`_lwp_start(f0983240, 0, 0, 0, 0, 0)
  Looking into it a bit, we see that this is an embedded blockpointer, so
  BP_GET_NDVAS should have returned 0:
       b32b240::blkptr
  EMBEDDED [L0 ZAP_OTHER] et=0 LZ4 size=200L/4aP birth=80L
  Instead, it looks like another thread is modifying this blockpointer:
       b32b240::ugrep | ::whatis
  f47a0e0c is in [ stack tid=0x19f ]
  ebd6ec40 is in [ stack tid=0x226 ]
  ea293bd0 is in [ stack tid=0x244 ]
  ea293be4 is in [ stack tid=0x244 ]

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>

git-svn-id: svn://svn.freebsd.org/base/stable/10@307270 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f