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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

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
> structure.
>
> 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.
>
> Result:
>   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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