Date:   Sat, 8 Jun 2019 09:06:20 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        andrea.parri@...rulasolutions.com, peterz@...radead.org
Subject: Re: [PATCH 1/2] btrfs: Implement DRW lock

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@...e.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.

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.

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?

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

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