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Subject: Re: [PATCH 1/2] btrfs: Implement DRW lock
To:     paulmck@linux.ibm.com
Cc:     linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        andrea.parri@amarulasolutions.com, peterz@infradead.org
References: <20190606135219.1086-1-nborisov@suse.com>
 <20190606135219.1086-2-nborisov@suse.com>
 <20190607105251.GB28207@linux.ibm.com>
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 <20190608151345.GC28207@linux.ibm.com>
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 <20190608160620.GH28207@linux.ibm.com>
From:   Nikolay Borisov <nborisov@suse.com>
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Date:   Sat, 8 Jun 2019 19:21:53 +0300
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On 8.06.19 г. 19:06 ч., Paul E. McKenney wrote:
> On Sat, Jun 08, 2019 at 06:44:17PM +0300, Nikolay Borisov wrote:
>> On 8.06.19 г. 18:13 ч., Paul E. McKenney wrote:
>>> On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
>>>> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
>>>>> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
>>>>>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
>>>>>> to have multiple readers or multiple writers but not multiple readers
>>>>>> and writers holding it concurrently. The code is factored out from
>>>>>> the existing open-coded locking scheme used to exclude pending
>>>>>> snapshots from nocow writers and vice-versa. Current implementation
>>>>>> actually favors Readers (that is snapshot creaters) to writers (nocow
>>>>>> writers of the filesystem).
>>>>>>
>>>>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>>>>>
>>>>> A preliminary question...
>>>>>
>>>>> What prevents the following sequence of events from happening?
>>>>>
>>>>> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
>>>>> 	which sees that lock->readers is zero and thus executes
>>>>> 	percpu_counter_inc(&lock->writers).
>>>>>
>>>>> o	btrfs_drw_read_lock() increments lock->readers, does the
>>>>> 	smp_mb__after_atomic(), and then does the wait_event().
>>>>> 	Because btrfs_drw_try_write_lock() incremented its CPU's
>>>>> 	lock->writers, the sum is the value one, so it blocks.
>>>>>
>>>>> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
>>>>> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
>>>>> 	(which decrements the current CPU's counter, so that a future
>>>>> 	sum would get zero), and returns false.
>>>>
>>>> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
>>>> already pointed that out and I've fixed it.
>>>
>>> Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?
> 
> Right...  Make that kernel/locking/locktorture.c
> 
>> At the moment - nothing. I rely on the fact that the original code I
>> extracted that from is bug-free (ha-ha). So perhahps hooking up
>> locktorture seems like a good suggestion. From a quick look I guess I
>> could mostly model that lock against the rwsem. The question is how do I
>> model the trylock semantics as well as the "double" part?
> 
> Implementing a correct synchronization primitive is like committing the
> perfect crime.  There are at least 50 things that can go wrong, and if
> you are a highly experienced genius, you -might- be able to anticipate
> and handle 25 of them.  (With apologies to any Kathleen Turner fans who
> might still be alive.)  Please note that this still applies to code
> ported from somewhere else because different environments likely have
> different assumptions and properties.

I agree, I'm far from thinking that the locking scheme is actually bug
free (hence the 'ha-ha') I'm not that arrogant (yet).

> 
> Therefore, heavy-duty stress testing is not optional.  In fact, formal
> verification is becoming non-optional as well -- please see Catalin
> Marinas's work on verifying the Linux kernel's queued spinlock for
> an example.

I assume you are referring to "Formal Methods for kernel hackers"? If
so, TLA+ has been on my radar ever since
https://lamport.azurewebsites.net/tla/formal-methods-amazon.pdf .

However I've yet to invest the time to be able to properly model a real
protocol (be it locking or otherwise) in it. Perhahps I could use the
DRW lock as a learning opportunity, we'll see.

> 
> You are right, current locktorture would get upset about having concurrent
> writers.  To teach locktorture about this, I suggest adding a flag to
> the lock_torture_ops structure named something like concurrent_write,
> but hopefully shorter.  Then this flag can be used to disable the "only
> one writer" check in lock_torture_writer().
> 
> Seem reasonable?

Good idea, I'll see to extending lock-torture but this will happen in a
week or so because I'm about to go on a holiday.

> 
> 							Thanx, Paul
> 
>>>> The idea here is that if a reader came after we've incremented out
>>>> percpu counter then it would have blocked, the writer would see that and
>>>> invoke btrfs_drw_write_unlock which will decrement the percpu counter
>>>> and will wakeup the reader that is now blocked on pending_readers.
>>>
>>> OK, I will await your next version.
>>>
>>> 							Thanx, Paul
>>>
>>>>> o	btrfs_drw_write_lock() therefore does its wait_event().
>>>>> 	Because lock->readers is nonzero, it blocks.
>>>>>
>>>>> o	Both tasks are now blocked.  In the absence of future calls
>>>>> 	to these functions (and perhaps even given such future calls),
>>>>> 	we have deadlock.
>>>>>
>>>>> So what am I missing here?
>>>>>
>>>>> 							Thanx, Paul
>>>>>
>>>>>> ---
>>>>>>  fs/btrfs/Makefile   |  2 +-
>>>>>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>>>>>>  3 files changed, 95 insertions(+), 1 deletion(-)
>>>>>>  create mode 100644 fs/btrfs/drw_lock.c
>>>>>>  create mode 100644 fs/btrfs/drw_lock.h
>>>>>>
>>>>>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
>>>>>> index ca693dd554e9..dc60127791e6 100644
>>>>>> --- a/fs/btrfs/Makefile
>>>>>> +++ b/fs/btrfs/Makefile
>>>>>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>>>>>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>>>>>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>>>>>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
>>>>>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
>>>>>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>>>>>>  
>>>>>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>>>>>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
>>>>>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
>>>>>> new file mode 100644
>>>>>> index 000000000000..9681bf7544be
>>>>>> --- /dev/null
>>>>>> +++ b/fs/btrfs/drw_lock.c
>>>>>> @@ -0,0 +1,71 @@
>>>>>> +#include "drw_lock.h"
>>>>>> +#include "ctree.h"
>>>>>> +
>>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	atomic_set(&lock->readers, 0);
>>>>>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
>>>>>> +	init_waitqueue_head(&lock->pending_readers);
>>>>>> +	init_waitqueue_head(&lock->pending_writers);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	percpu_counter_destroy(&lock->writers);
>>>>>> +}
>>>>>> +
>>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	if (atomic_read(&lock->readers))
>>>>>> +		return false;
>>>>>> +
>>>>>> +	percpu_counter_inc(&lock->writers);
>>>>>> +
>>>>>> +	/*
>>>>>> +	 * Ensure writers count is updated before we check for
>>>>>> +	 * pending readers
>>>>>> +	 */
>>>>>> +	smp_mb();
>>>>>> +	if (atomic_read(&lock->readers)) {
>>>>>> +		btrfs_drw_read_unlock(lock);
>>>>>> +		return false;
>>>>>> +	}
>>>>>> +
>>>>>> +	return true;
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	while(true) {
>>>>>> +		if (btrfs_drw_try_write_lock(lock))
>>>>>> +			return;
>>>>>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
>>>>>> +	}
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	percpu_counter_dec(&lock->writers);
>>>>>> +	cond_wake_up(&lock->pending_readers);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	atomic_inc(&lock->readers);
>>>>>> +	smp_mb__after_atomic();
>>>>>> +
>>>>>> +	wait_event(lock->pending_readers,
>>>>>> +		   percpu_counter_sum(&lock->writers) == 0);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	/*
>>>>>> +	 * Atomic RMW operations imply full barrier, so woken up writers
>>>>>> +	 * are guaranteed to see the decrement
>>>>>> +	 */
>>>>>> +	if (atomic_dec_and_test(&lock->readers))
>>>>>> +		wake_up(&lock->pending_writers);
>>>>>> +}
>>>>>> +
>>>>>> +
>>>>>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
>>>>>> new file mode 100644
>>>>>> index 000000000000..baff59561c06
>>>>>> --- /dev/null
>>>>>> +++ b/fs/btrfs/drw_lock.h
>>>>>> @@ -0,0 +1,23 @@
>>>>>> +#ifndef BTRFS_DRW_LOCK_H
>>>>>> +#define BTRFS_DRW_LOCK_H
>>>>>> +
>>>>>> +#include <linux/atomic.h>
>>>>>> +#include <linux/wait.h>
>>>>>> +#include <linux/percpu_counter.h>
>>>>>> +
>>>>>> +struct btrfs_drw_lock {
>>>>>> +	atomic_t readers;
>>>>>> +	struct percpu_counter writers;
>>>>>> +	wait_queue_head_t pending_writers;
>>>>>> +	wait_queue_head_t pending_readers;
>>>>>> +};
>>>>>> +
>>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
>>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
>>>>>> +
>>>>>> +#endif
>>>>>> -- 
>>>>>> 2.17.1
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>
> 
>