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Re: [Xen-devel] [PATCH V5] x86 spinlock: Fix memory corruption on completing completions

On 02/15/2015 11:25 AM, Raghavendra K T wrote:
Paravirt spinlock clears slowpath flag after doing unlock.
As explained by Linus currently it does:
                 prev = *lock;
                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);

                 /* add_smp() is a full mb() */

                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
                         __ticket_unlock_slowpath(lock, prev);

which is *exactly* the kind of things you cannot do with spinlocks,
because after you've done the "add_smp()" and released the spinlock
for the fast-path, you can't access the spinlock any more.  Exactly
because a fast-path lock might come in, and release the whole data

Linus suggested that we should not do any writes to lock after unlock(),
and we can move slowpath clearing to fastpath lock.

So this patch implements the fix with:
1. Moving slowpath flag to head (Oleg):
Unlocked locks don't care about the slowpath flag; therefore we can keep
it set after the last unlock, and clear it again on the first (try)lock.
-- this removes the write after unlock. note that keeping slowpath flag would
result in unnecessary kicks.
By moving the slowpath flag from the tail to the head ticket we also avoid
the need to access both the head and tail tickets on unlock.

2. use xadd to avoid read/write after unlock that checks the need for
unlock_kick (Linus):
We further avoid the need for a read-after-release by using xadd;
the prev head value will include the slowpath flag and indicate if we
need to do PV kicking of suspended spinners -- on modern chips xadd
isn't (much) more expensive than an add + load.

  setup: 16core (32 cpu +ht sandy bridge 8GB 16vcpu guest)
  benchmark overcommit %improve
  kernbench  1x           -0.13
  kernbench  2x            0.02
  dbench     1x           -1.77
  dbench     2x           -0.63

[Jeremy: hinted missing TICKET_LOCK_INC for kick]
[Oleg: Moving slowpath flag to head, ticket_equals idea]
[PeterZ: Detailed changelog]

Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>

Sasha, Hope this addresses invalid read concern you had with latest
xadd based implementation.

(Think we need to test without Oleg's PATCH] sched/completion: completion_done() should serialize with complete() reported by Paul.)

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