drm/xe/lrc: document sentinel and make CTX_TIMESTAMP read TOCTOU-safe

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Problem: CTX_TIMESTAMP MMIO reads could be stale if a context
switched out between check and read; 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.

Fixes: d243ef6a39 ("drm/xe/lrc: Refactor xe_lrc_timestamp to simplify logic")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/7956
Assisted-by: GitHub-Copilot:claude-sonnet-5
Signed-off-by: Gajendra Uttamchand <gajendra.uttamchand@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patch.msgid.link/20260810071812.213358-4-gajendra.uttamchand@intel.com
(cherry picked from commit a806534474)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

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Gajendra Uttamchand 2026-08-10 07:18:14 +00:00 • committed by Thomas Hellström

parent 51afaf53e0

commit cb4afddf9e

2 changed files with 21 additions and 8 deletions

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22

drivers/gpu/drm/xe/xe_lrc.c

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7

drivers/gpu/drm/xe/xe_lrc.h

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