From: Valentin Schneider <valentin.schneider@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Nikolay Borisov <nborisov@suse.com>,
linux-btrfs@vger.kernel.org, paulmck@linux.ibm.com,
andrea.parri@amarulasolutions.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] Refactor snapshot vs nocow writers locking
Date: Tue, 30 Jul 2019 12:03:34 +0100 [thread overview]
Message-ID: <69ef76a2-ebd6-956e-c611-2e742606ed95@arm.com> (raw)
In-Reply-To: <60eda0ab-08b3-de82-5b06-98386ee1928f@arm.com>
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/
next prev parent reply other threads:[~2019-07-30 11:03 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-19 8:39 [PATCH v2 0/2] Refactor snapshot vs nocow writers locking Nikolay Borisov
2019-07-19 8:39 ` [PATCH v2 1/2] btrfs: Implement DRW lock Nikolay Borisov
2019-07-19 8:39 ` [PATCH v2 2/2] btrfs: convert snapshot/nocow exlcusion to drw lock Nikolay Borisov
2019-07-19 8:48 ` [RFC PATCH] btrfs: Hook btrfs' DRW lock to locktorture infrastructure Nikolay Borisov
2019-08-05 16:36 ` Nathan Chancellor
2019-08-05 18:17 ` David Sterba
2019-07-29 14:13 ` [PATCH v2 0/2] Refactor snapshot vs nocow writers locking Valentin Schneider
2019-07-29 15:33 ` Catalin Marinas
2019-07-29 16:32 ` Valentin Schneider
2019-07-30 11:03 ` Valentin Schneider [this message]
2019-07-30 12:11 ` Nikolay Borisov
2019-07-30 13:36 ` Valentin Schneider
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