Messages in this thread | | | Subject | Re: [PATCH v2 0/2] Refactor snapshot vs nocow writers locking | From | Valentin Schneider <> | Date | Tue, 30 Jul 2019 12:03:34 +0100 |
| |
On 29/07/2019 17:32, Valentin Schneider wrote: > On 29/07/2019 16:33, Catalin Marinas wrote: [...] >> I'd say that's one of the pitfalls of PlusCal. The above is executed >> atomically, so you'd have the lock_state read and updated in the same >> action. Looking at the C patches, there is an >> atomic_read(&lock->readers) followed by a >> percpu_counter_inc(&lock->writers). Between these two, you can have >> "readers" becoming non-zero via a different CPU. >> >> My suggestion would be to use procedures with labels to express the >> non-atomicity of such sequences. >> >
FYI, with a very simple and stupid modification of the spec:
----->8----- macro ReadUnlock() { reader_count := reader_count - 1; \* Condition variable signal is "implicit" here }
macro WriteUnlock() { writer_count := writer_count - 1; \* Ditto on the cond var }
procedure ReadLock() { add: reader_count := reader_count + 1; lock: await writer_count = 0; return; }
procedure WriteLock() { add: writer_count := writer_count + 1; lock: await reader_count = 0; return; }; -----8<-----
it's quite easy to trigger the case Paul pointed out in [1]:
----->8----- Error: Deadlock reached. Error: The behavior up to this point is: State 1: <Initial predicate> /\ stack = (<<reader, 1>> :> <<>> @@ <<writer, 1>> :> <<>>) /\ pc = (<<reader, 1>> :> "loop" @@ <<writer, 1>> :> "loop_") /\ writer_count = 0 /\ reader_count = 0 /\ lock_state = "idle"
State 2: <loop_ line 159, col 16 to line 164, col 72 of module specs> /\ stack = ( <<reader, 1>> :> <<>> @@ <<writer, 1>> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) /\ pc = (<<reader, 1>> :> "loop" @@ <<writer, 1>> :> "add") /\ writer_count = 0 /\ reader_count = 0 /\ lock_state = "idle"
State 3: <add line 146, col 14 to line 149, col 63 of module specs> /\ stack = ( <<reader, 1>> :> <<>> @@ <<writer, 1>> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) /\ pc = (<<reader, 1>> :> "loop" @@ <<writer, 1>> :> "lock") /\ writer_count = 1 /\ reader_count = 0 /\ lock_state = "idle"
State 4: <loop line 179, col 15 to line 184, col 71 of module specs> /\ stack = ( <<reader, 1>> :> <<[pc |-> "read_cs", procedure |-> "ReadLock"]>> @@ <<writer, 1>> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) /\ pc = (<<reader, 1>> :> "add_" @@ <<writer, 1>> :> "lock") /\ writer_count = 1 /\ reader_count = 0 /\ lock_state = "idle"
State 5: <add_ line 133, col 15 to line 136, col 64 of module specs> /\ stack = ( <<reader, 1>> :> <<[pc |-> "read_cs", procedure |-> "ReadLock"]>> @@ <<writer, 1>> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) /\ pc = (<<reader, 1>> :> "lock_" @@ <<writer, 1>> :> "lock") /\ writer_count = 1 /\ reader_count = 1 /\ lock_state = "idle" -----8<-----
Which I think is pretty cool considering the effort that was required (read: not much).
[1]: https://lore.kernel.org/lkml/20190607105251.GB28207@linux.ibm.com/
| |