Re: [PATCH i-g-t 1/2] lib/xe/xe_spin: initialize ticks_delta to ~0u

On 7/29/2026 11:42 AM, Zbigniew Kempczyński wrote:
> On Tue, Jul 28, 2026 at 12:26:58PM +0200, Marcin Bernatowicz wrote:
>> Initialize ticks_delta to ~0u instead of 0. This value does not satisfy
>> the COND_BBE exit condition and prevents false batch termination.
> May you elaborate? Does it mean STORE-DWORD (loop) is not visible in the memory
> before COND_BBE is executed? IIUC with ~0 we could also drop this loop.
The issue is not about the STORE-DWORDs (loop) visibility within a 
single batch; that's what the pad is for and that path works fine.
The problem is a cross-iteration potential stale cache read: when the 
same BO is re-submitted, COND_BBE can read the ticks_delta value leftover
from the previous batch execution - which is <= ctx_ticks (since that's 
the exit condition) - and immediately exit before the new SRM write 
propagates.

With ticks_delta = 0 (or any value <= ctx_ticks at prev-iteration exit), 
a stale read satisfies COND_BBE.
With ~0u, even a stale read of the sentinel gives 0xFFFFFFFF <= 
ctx_ticks -> false -> batch continues safely.

To reproduce: each gem_wsim invocation does 4000 submit/wait cycles on 
the same BO (5 engines × 5ms each).
The failure appears only after many outer iterations under sustained load:

i=1; while true; do
     echo "Iteration: $i"
     gem_wsim -w 
"1.RCS.5000.0.0,1.BCS.5000.0.0,1.CCS.5000.0.1,1.VCS.5000.0.0,1.VECS.5000.0.0" 
-VV -r 4000 || break
     ((i++))
done

Typical failure after ~60 outer iterations:

CRITICAL: Failed assertion: w->duration.requested_ticks <= 
~w->xe.data->spin.ticks_delta
CRITICAL: error: 96000 > 2 (sometimes 0, 1)

With the sentinel reset before each submission, ran two days without 
failure.

--

Marcin

>
> --
> Zbigniew
>
>> Signed-off-by: Marcin Bernatowicz <[email protected]>
>> Cc: Adam Miszczak <[email protected]>
>> Cc: Kamil Konieczny <[email protected]>
>> Cc: Lukasz Laguna <[email protected]>
>> ---
>>   lib/xe/xe_spin.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/xe/xe_spin.c b/lib/xe/xe_spin.c
>> index 874310789..331bed1d2 100644
>> --- a/lib/xe/xe_spin.c
>> +++ b/lib/xe/xe_spin.c
>> @@ -60,7 +60,7 @@ void xe_spin_init(struct xe_spin *spin, struct xe_spin_opts *opts)
>>   	spin->start = 0;
>>   	spin->end = 0xffffffff;
>>   	spin->wait_cond = 0;
>> -	spin->ticks_delta = 0;
>> +	spin->ticks_delta = ~0u;
>>   
>>   	if (opts->ctx_ticks) {
>>   		/* store start timestamp */
>> -- 
>> 2.43.0
>>