os/bluestore: simplify and fix SharedBlob::put()

There is a narrow race possible:

A: lookup foo
A: put on foo
A:   foo --nref == 0
B: lookup foo
B: put foo
B:   foo --nref == 0
B: try_remove() succeeds, removes
A: try_remove() tries to remove foo again, probably crashes

We could fix this by flagging the object in some way to indicate it was
removed (maybe clearing parent?), but then we need to be careful about
dereferencing foo to get parent from put().

Fix this by moving to a simpler model: make lookup fail if nref == 0.
This eliminates the races around put() entirely because once nref reaches
0 it never goes up again.

Fixes: http://tracker.ceph.com/issues/24211
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 8c8944b2c45ca9dc5b8fd4db1590e1d24206c0b3)

diff --git a/src/os/bluestore/BlueStore.cc b/src/os/bluestore/BlueStore.cc
index b1cfed2651e94fe71af55a49407932cf4974e0ad..17e485a1319325609d034add072c5f22a4cdf43d 100644 (file)
--- a/src/os/bluestore/BlueStore.cc
+++ b/src/os/bluestore/BlueStore.cc
@@ -1671,16 +1671,9 @@ void BlueStore::SharedBlob::put()
                             << " removing self from set " << get_parent()
                             << dendl;
     if (get_parent()) {
-      if (get_parent()->try_remove(this)) {
-       delete this;
-      } else {
-       ldout(coll->store->cct, 20)
-         << __func__ << " " << this << " lost race to remove myself from set"
-         << dendl;
-      }
-    } else {
-      delete this;
+      get_parent()->remove_last(this);
     }
+    delete this;
   }
 }
 

diff --git a/src/os/bluestore/BlueStore.h b/src/os/bluestore/BlueStore.h
index a1e10712a597b078a88bf67bf09e626dd05dd4d7..b2048584b01c3ad4c439efc73865d5ff176308f9 100644 (file)
--- a/src/os/bluestore/BlueStore.h
+++ b/src/os/bluestore/BlueStore.h
@@ -437,7 +437,8 @@ public:
     SharedBlobRef lookup(uint64_t sbid) {
       std::lock_guard<std::mutex> l(lock);
       auto p = sb_map.find(sbid);
-      if (p == sb_map.end()) {
+      if (p == sb_map.end() ||
+         p->second->nref == 0) {
         return nullptr;
       }
       return p->second;
@@ -449,14 +450,11 @@ public:
       sb->coll = coll;
     }
 
-    bool try_remove(SharedBlob *sb) {
+    void remove_last(SharedBlob *sb) {
       std::lock_guard<std::mutex> l(lock);
-      if (sb->nref == 0) {
-       assert(sb->get_parent() == this);
-       sb_map.erase(sb->get_sbid());
-       return true;
-      }
-      return false;
+      assert(sb->nref == 0);
+      assert(sb->get_parent() == this);
+      sb_map.erase(sb->get_sbid());
     }
 
     void remove(SharedBlob *sb) {