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

Sasha Levin
2015-02-17T18:27:54Z

On 02/15/2015 01:01 AM, Raghavendra K T wrote:
> 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@oracle.com>
>> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
>> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>> ---
>
> 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.)
>

I ran it for a while and everything seems to work correctly:

Tested-by: Sasha Levin <sasha.levin@oracle.com>

Thanks,
Sasha