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SubjectRe: [PATCH 5/5] fs/locks: create a tree of dependent requests.
On Thu, Aug 09 2018, J. Bruce Fields wrote:

> On Thu, Aug 09, 2018 at 12:04:41PM +1000, NeilBrown wrote:
>> When we find an existing lock which conflicts with a request,
>> and the request wants to wait, we currently add the request
>> to a list. When the lock is removed, the whole list is woken.
>> This can cause the thundering-herd problem.
>> To reduce the problem, we make use of the (new) fact that
>> a pending request can itself have a list of blocked requests.
>> When we find a conflict, we look through the existing blocked requests.
>> If any one of them blocks the new request, the new request is attached
>> below that request.
>> This way, when the lock is released, only a set of non-conflicting
>> locks will be woken. The rest of the herd can stay asleep.
>
> That that's not true any more--some of the locks you wake may conflict
> with each other. Is that right? Which is fine (the possibility of
> thundering herds in weird overlapping-range cases probably isn't a big
> deal). I just want to make sure I understand....

Yes, you are correct.
Lock waiters will be woken if they were directly blocked by a lock that
has been released, if they were blocked (directly or indirectly) by a
lock which is now blocked by a lock that they don't conflict with.
The first set will be mutually non-conflicting.

>
> I think you could simplify the code a lot by maintaining the tree so
> that it always satisfies the condition that waiters are always strictly
> "weaker" than their descendents, so that finding a conflict with a
> waiter is always enough to know that the descendents also conflict.

Can you define "weaker" please.
I suspect it is a partial ordering, in which case a tree would normally
be more appropriate than trying to find a total ordering.

Thanks,
NeilBrown

>
> So, when you put a waiter to sleep, you don't add it below a child
> unless it's "stronger" than the child.
>
> You give up the property that siblings don't conflict, but again that
> just means thundering herds in weird cases, which is OK.
>
> --b.
>
>>
>> Reported-and-tested-by: Martin Wilck <mwilck@suse.de>
>> Signed-off-by: NeilBrown <neilb@suse.com>
>> ---
>> fs/locks.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
>> 1 file changed, 63 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/locks.c b/fs/locks.c
>> index fc64016d01ee..17843feb6f5b 100644
>> --- a/fs/locks.c
>> +++ b/fs/locks.c
>> @@ -738,6 +738,39 @@ static void locks_delete_block(struct file_lock *waiter)
>> spin_unlock(&blocked_lock_lock);
>> }
>>
>> +static void wake_non_conflicts(struct file_lock *waiter, struct file_lock *blocker,
>> + enum conflict conflict(struct file_lock *,
>> + struct file_lock *))
>> +{
>> + struct file_lock *parent = waiter;
>> + struct file_lock *fl;
>> + struct file_lock *t;
>> +
>> + fl = list_entry(&parent->fl_blocked, struct file_lock, fl_block);
>> +restart:
>> + list_for_each_entry_safe_continue(fl, t, &parent->fl_blocked, fl_block) {
>> + switch (conflict(fl, blocker)) {
>> + default:
>> + case FL_NO_CONFLICT:
>> + __locks_wake_one(fl);
>> + break;
>> + case FL_CONFLICT:
>> + /* Need to check children */
>> + parent = fl;
>> + fl = list_entry(&parent->fl_blocked, struct file_lock, fl_block);
>> + goto restart;
>> + case FL_TRANSITIVE_CONFLICT:
>> + /* all children must also conflict, no need to check */
>> + continue;
>> + }
>> + }
>> + if (parent != waiter) {
>> + parent = parent->fl_blocker;
>> + fl = parent;
>> + goto restart;
>> + }
>> +}
>> +
>> /* Insert waiter into blocker's block list.
>> * We use a circular list so that processes can be easily woken up in
>> * the order they blocked. The documentation doesn't require this but
>> @@ -747,11 +780,32 @@ static void locks_delete_block(struct file_lock *waiter)
>> * fl_blocked list itself is protected by the blocked_lock_lock, but by ensuring
>> * that the flc_lock is also held on insertions we can avoid taking the
>> * blocked_lock_lock in some cases when we see that the fl_blocked list is empty.
>> + *
>> + * Rather than just adding to the list, we check for conflicts with any existing
>> + * waiter, and add to that waiter instead.
>> + * Thus wakeups don't happen until needed.
>> */
>> static void __locks_insert_block(struct file_lock *blocker,
>> - struct file_lock *waiter)
>> + struct file_lock *waiter,
>> + enum conflict conflict(struct file_lock *,
>> + struct file_lock *))
>> {
>> + struct file_lock *fl;
>> BUG_ON(!list_empty(&waiter->fl_block));
>> +
>> + /* Any request in waiter->fl_blocked is know to conflict with
>> + * waiter, but it might not conflict with blocker.
>> + * If it doesn't, it needs to be woken now so it can find
>> + * somewhere else to wait, or possible it can get granted.
>> + */
>> + if (conflict(waiter, blocker) != FL_TRANSITIVE_CONFLICT)
>> + wake_non_conflicts(waiter, blocker, conflict);
>> +new_blocker:
>> + list_for_each_entry(fl, &blocker->fl_blocked, fl_block)
>> + if (conflict(fl, waiter)) {
>> + blocker = fl;
>> + goto new_blocker;
>> + }
>> waiter->fl_blocker = blocker;
>> list_add_tail(&waiter->fl_block, &blocker->fl_blocked);
>> if (IS_POSIX(blocker) && !IS_OFDLCK(blocker))
>> @@ -760,10 +814,12 @@ static void __locks_insert_block(struct file_lock *blocker,
>>
>> /* Must be called with flc_lock held. */
>> static void locks_insert_block(struct file_lock *blocker,
>> - struct file_lock *waiter)
>> + struct file_lock *waiter,
>> + enum conflict conflict(struct file_lock *,
>> + struct file_lock *))
>> {
>> spin_lock(&blocked_lock_lock);
>> - __locks_insert_block(blocker, waiter);
>> + __locks_insert_block(blocker, waiter, conflict);
>> spin_unlock(&blocked_lock_lock);
>> }
>>
>> @@ -1033,7 +1089,7 @@ static int flock_lock_inode(struct inode *inode, struct file_lock *request)
>> if (!(request->fl_flags & FL_SLEEP))
>> goto out;
>> error = FILE_LOCK_DEFERRED;
>> - locks_insert_block(fl, request);
>> + locks_insert_block(fl, request, flock_locks_conflict);
>> goto out;
>> }
>> if (request->fl_flags & FL_ACCESS)
>> @@ -1107,7 +1163,8 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
>> spin_lock(&blocked_lock_lock);
>> if (likely(!posix_locks_deadlock(request, fl))) {
>> error = FILE_LOCK_DEFERRED;
>> - __locks_insert_block(fl, request);
>> + __locks_insert_block(fl, request,
>> + posix_locks_conflict);
>> }
>> spin_unlock(&blocked_lock_lock);
>> goto out;
>> @@ -1581,7 +1638,7 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
>> break_time -= jiffies;
>> if (break_time == 0)
>> break_time++;
>> - locks_insert_block(fl, new_fl);
>> + locks_insert_block(fl, new_fl, leases_conflict);
>> trace_break_lease_block(inode, new_fl);
>> spin_unlock(&ctx->flc_lock);
>> percpu_up_read_preempt_enable(&file_rwsem);
>>
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