Re: [PATCH] locking/pvqspinlock: Use try_cmpxchg() in pv_unhash

[Waiman Long <llong@redhat.com>]

On 12/23/24 2:47 AM, Bibo Mao wrote:
> We ported pv spinlock to old linux kernel on LoongArch platform, there is
What old kernel are you using? Race condition like that should be hard 
to reproduce. Do you hit this bug only once?
> error with some stress tests. The error report is something like this for
> short:
>   kernel BUG at kernel/locking/qspinlock_paravirt.h:261!
>   Oops - BUG[#1]:
>   CPU: 1 PID: 6613 Comm: pidof Not tainted 4.19.190+ #43
>   Hardware name: Loongson KVM, BIOS 0.0.0 02/06/2015
>      ra: 9000000000509cfc do_task_stat+0x29c/0xaf0
>     ERA: 9000000000291308 __pv_queued_spin_unlock_slowpath+0xf8/0x100
>    CRMD: 000000b0 (PLV0 -IE -DA +PG DACF=CC DACM=CC -WE)
>    PRMD: 00000000 (PPLV0 -PIE -PWE)
>           ...
>   Call Trace:
>   [<9000000000291308>] __pv_queued_spin_unlock_slowpath+0xf8/0x100
>   [<9000000000509cf8>] do_task_stat+0x298/0xaf0
>   [<9000000000502570>] proc_single_show+0x60/0xe0
>
> The problem is that memory accessing is out of order on LoongArch
> platform, there is contension between pv_unhash() and pv_hash().
>
> CPU0 pv_unhash:                                CPU1 pv_hash:
>
>    for_each_hash_entry(he, offset, hash) {        for_each_hash_entry(he, offset, hash) {
>      if (READ_ONCE(he->lock) == lock) {             struct qspinlock *old = NULL;
>        node = READ_ONCE(he->node);
>        WRITE_ONCE(he->lock, NULL);
>
> On LoongArch platform which is out of order, the execution order may be
> switched like this:
>>       WRITE_ONCE(he->lock, NULL);
>                                                      if (try_cmpxchg(&he->lock, &old, lock)) {
>                                                        WRITE_ONCE(he->node, node);
>                                                        return &he->lock;
>
> CPU1 pv_hash() is executing and watch that lock is set with NULL. Write
> he->node with node of new lock.
>>       node = READ_ONCE(he->node);
> READ_ONCE(he->node) on CPU0 will return node of new lock rather than itself.
The pv_hash_entry structure is supposed to be cacheline aligned. The 
use  of READ_ONCE/WRITE_ONCE will ensure that compiler won't rearrange 
the ordering. If the CPU can rearrange read/write ordering on the same 
cacheline like that, it may be some advance optimization technique that 
I don't  quite understand. Can you check if the buffer returned by 
alloc_large_system_hash() is really properly aligned?
>
> Here READ_ONCE/WRITE_ONCE is replaced with try_cmpxchg().
>
> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
> ---
>   kernel/locking/qspinlock_paravirt.h | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index dc1cb90e3644..cc4eabce092d 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -240,9 +240,10 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
>   	struct pv_node *node;
>   
>   	for_each_hash_entry(he, offset, hash) {
> -		if (READ_ONCE(he->lock) == lock) {
> +		struct qspinlock *old = lock;
> +
> +		if (try_cmpxchg(&he->lock, &old, NULL)) {
>   			node = READ_ONCE(he->node);
> -			WRITE_ONCE(he->lock, NULL);
>   			return node;
>   		}
>   	}

Anyway, this change isn't quite right as suggested by Akira.

Cheers,
Longman

>
> base-commit: 48f506ad0b683d3e7e794efa60c5785c4fdc86fa