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.gb28...@linux.ibm.com/
