Messages in this thread | | | Subject | Re: [PATCH v2 0/2] Refactor snapshot vs nocow writers locking | From | Nikolay Borisov <> | Date | Tue, 30 Jul 2019 15:11:46 +0300 |
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On 30.07.19 г. 14:03 ч., Valentin Schneider wrote: > 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]:
Yes, however, there was a bug in the original posting, in that btrfs_drw_try_write_lock should have called btrfs_drw_write_unlock instead of btrfs_drw_read_unlock if it sees that readers incremented while it has already incremented its percpu counter.
Additionally the implementation doesn't await with the respective variable incremented. Is there a way to express something along the lines of :
> procedure WriteLock() > { > add: > writer_count := writer_count + 1; > lock: > await reader_count = 0;
If we are about to wait then also decrement writer_count? I guess the correct way to specify it would be:
procedure WriteLock() {
writer_count := writer_count + 1; await reader_count = 0; return; };
Because the implementation (by using barriers and percpu counters ensures all of this happens as one atomic step?) E.g. before going to sleep we decrement the write unlock.
> return; > };
> > ----->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/ >
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