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LRC stores a sentinel while > > a context starts that must not be treated as a real timestamp. > > > > Fix: Check the LRC-stored sentinel before and after the MMIO read; > > return the LRC value if the context switched out to avoid TOCTOU. > > > > Note: Keep XE_LRC_CTX_TIMESTAMP_ACTIVE in xe_lrc.h as the > > canonical sentinel. > > > > Signed-off-by: Uttamchand, Gajendra > > Hi Gajendra, > > Thanks for addressing this. > > The patch looks correct. I think this regression was introduced when refactoring for multi-queue, so please add a Fixes tag to this. > > Fixes: d243ef6a39c6 ("drm/xe/lrc: Refactor xe_lrc_timestamp to simplify logic") > > With that, this is > > Reviewed-by: Umesh Nerlige Ramappa > Sashiko [1] also flagged some other valid existing issue. There are very hard to hit race, but look valid. I don't think it worth blocking this patch and this fix itself LGTM but I think we should open a Jira to track the issues raised by Sashiko and get those fixed sooner or later. With the fixes tag Umesh suggests: Acked-by: Matthew Brost [1] https://sashiko.dev/#/patchset/20260612032745.179673-4-gajendra.uttamchand%40intel.com > Umesh > > > --- > > drivers/gpu/drm/xe/xe_lrc.c | 29 +++++++++++++++++++++-------- > > drivers/gpu/drm/xe/xe_lrc.h | 7 +++++++ > > 2 files changed, 28 insertions(+), 8 deletions(-) > > > > diff --git a/drivers/gpu/drm/xe/xe_lrc.c b/drivers/gpu/drm/xe/xe_lrc.c > > index a4292a11391d..e284c008c126 100644 > > --- a/drivers/gpu/drm/xe/xe_lrc.c > > +++ b/drivers/gpu/drm/xe/xe_lrc.c > > @@ -1096,7 +1096,7 @@ static void xe_lrc_finish(struct xe_lrc *lrc) > > * on until it is scheduled, we also read the ENGINE_ID MMIO in the WA BB and > > * store it in the PPHSWP. > > */ > > -#define CONTEXT_ACTIVE 1ULL > > +#define CONTEXT_ACTIVE XE_LRC_CTX_TIMESTAMP_ACTIVE > > static ssize_t setup_utilization_wa(struct xe_lrc *lrc, > > struct xe_hw_engine *hwe, > > u32 *batch, > > @@ -2720,21 +2720,34 @@ static u64 xe_lrc_update_multi_queue_timestamp(struct xe_lrc *lrc, u64 *old_ts) > > static u64 xe_lrc_context_timestamp(struct xe_lrc *lrc) > > { > > u64 reg_ts, new_ts = lrc->ctx_timestamp; > > + u64 stored; > > > > /* CTX_TIMESTAMP mmio read is invalid on VF, so return the LRC value */ > > if (IS_SRIOV_VF(lrc_to_xe(lrc))) > > return xe_lrc_ctx_timestamp(lrc); > > > > - if (context_active(lrc) && > > - !get_ctx_timestamp(lrc, xe_lrc_engine_id(lrc), ®_ts)) > > + /* > > + * Safely read CTX_TIMESTAMP: check the LRC-stored value before and > > + * after the MMIO read to avoid a TOCTOU where a context switch makes the > > + * MMIO value stale. If the LRC value is not `CONTEXT_ACTIVE` return it; > > + * otherwise accept the MMIO value only if the context remained active. > > + */ > > + > > + stored = xe_lrc_ctx_timestamp(lrc); > > + if (stored != CONTEXT_ACTIVE) > > + return stored; > > + > > + /* Context is active: read the live timestamp from the engine's MMIO register. */ > > + if (!get_ctx_timestamp(lrc, xe_lrc_engine_id(lrc), ®_ts)) > > new_ts = reg_ts; > > > > - /* > > - * If context swicthed out while we were here, just return the latest > > - * LRC CTX TIMESTAMP value. > > + /* Re-check the LRC-stored timestamp: if the context switched out while > > + * reading MMIO the hardware saved the canonical timestamp into the LRC > > + * during context-save, so return that value instead of the MMIO read. > > */ > > - if (!context_active(lrc)) > > - return xe_lrc_ctx_timestamp(lrc); > > + stored = xe_lrc_ctx_timestamp(lrc); > > + if (stored != CONTEXT_ACTIVE) > > + return stored; > > > > return new_ts; > > } > > diff --git a/drivers/gpu/drm/xe/xe_lrc.h b/drivers/gpu/drm/xe/xe_lrc.h > > index 0a3a611391ee..7be5e3da8bc8 100644 > > --- a/drivers/gpu/drm/xe/xe_lrc.h > > +++ b/drivers/gpu/drm/xe/xe_lrc.h > > @@ -9,6 +9,13 @@ > > > > #include "xe_lrc_types.h" > > > > +/* > > + * Sentinel value stored in lrc->ctx_timestamp while a context is starting. > > + * The hardware hasn't yet written the real CTX_TIMESTAMP, so this is not a > > + * valid elapsed-time sample and must not be used as one. > > + */ > > +#define XE_LRC_CTX_TIMESTAMP_ACTIVE 1ULL > > + > > struct drm_printer; > > struct xe_bb; > > struct xe_device; > > -- > > 2.43.0 > >