Re: [PATCH v2 0/2] Refactor snapshot vs nocow writers locking

[Valentin Schneider <valentin.schneider@arm.com>]

On 29/07/2019 16:33, Catalin Marinas wrote:
[...]
>> ---- MODULE specs ----
>> EXTENDS Integers, Sequences, TLC
>>
>> CONSTANTS
>>     NR_WRITERS,
>>     NR_READERS,
>>     WRITER_TASK,
>>     READER_TASK
>>
>> WRITERS == {WRITER_TASK} \X (1..NR_WRITERS)
>> READERS == {READER_TASK} \X (1..NR_READERS)
>> THREADS == WRITERS \union READERS
> 
> Recommendation: use symbolic values for WRITERS and READERS (defined in
> .cfg: e.g. r1, r2, r3, w1, w2, w2). It allows you do to symmetry
> optimisations. We've also hit a TLC bug in the past with process values
> made up of a Cartesian product (though it may have been fixed since).
> 

Right, I had forgotten that one:

  https://github.com/tlaplus/tlaplus/issues/164

Being very lazy I dislike having to manually input those, but as you say
it can't be avoided if we want to use symmetry.

>> macro ReadLock(tid)
>> {
>>     if (lock_state = "idle" \/ lock_state = "read_locked") {
>>         lock_state := "read_locked";
>>         threads[tid] := "read_locked";
>>     } else {
>>         assert lock_state = "write_locked";
>>         \* waiting for writers to finish
>>         threads[tid] := "write_waiting";
>>         await lock_state = "" \/ lock_state = "read_locked";
> 
> lock_state = "idle"?
> 

Aye, I didn't modify those macros from the original spec.

>> macro WriteLock(tid)
>> {
>>     if (lock_state = "idle" \/ lock_state = "write_locked") {
>>         lock_state := "write_locked";
>>         threads[tid] := "write_locked";
>>     } else {
>>         assert lock_state = "read_locked";
>>         \* waiting for readers to finish
>>         threads[tid] := "read_waiting";
>>         await lock_state = "idle" \/ lock_state = "write_locked";
>>     };
>> }
> 
> 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.
> 

Agreed, I've suggested something like this in my reply.

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