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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>
From:   Nikolay Borisov <nborisov@suse.com>
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Date:   Sat, 8 Jun 2019 18:44:17 +0300
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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?

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?

> 
>> 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
>>>>
>>>
>>>
>>
> 
>