Locking(9)

📦 version 7.3.1, in nixpkgs unstable

💅 Locking 💅

ok so bestie, welcome to Locking(9) from the Kernel API Manual — grab a lil snack, this one's about who's holding the lock (spoiler: not DRM GPUVM) 🫶

🔐 the whole vibe

so in terms of managing drm_gpuva entries, DRM GPUVM does not take care of locking itself — nope, that's literally the driver's responsibility, no cap. the driver is the one who has to worry about locking, and the ops it probably wants to protect are: inserting, removing and iterating drm_gpuva objects, plus generating all kinds of operations, such as split / merge or prefetch. ✨

DRM GPUVM also does not take care of the locking of the backing drm_gem_object buffers GPU VA lists and drm_gpuvm_bo abstractions by itself either — drivers are responsible to enforce mutual exclusion using either the GEMs dma_resv lock or the GEMs gpuva.lock mutex. pick your fighter 💅

🕵️ the lockdep checks (yes there are checks)

ok but here's the part that's giving accountability: DRM GPUVM does contain lockdep checks to ensure callers of its API hold the corresponding lock whenever the drm_gem_objects GPU VA list is accessed by functions such as drm_gpuva_link or drm_gpuva_unlink — but also drm_gpuvm_bo_obtain and drm_gpuvm_bo_put. 🔥

and the latter is required since on creation and destruction of a drm_gpuvm_bo the drm_gpuvm_bo is attached / removed from the drm_gem_objects gpuva list. subsequent calls to drm_gpuvm_bo_obtain for the same drm_gpuvm and drm_gem_object must be able to observe previous creations and destructions of drm_gpuvm_bos in order to keep instances unique. iykyk 👑

📋 the internal lists

the drm_gpuvm's lists for keeping track of external and evicted objects are protected against concurrent insertion / removal and iteration internally. slay, one less thing.

however — and yes there's a however, there's always a however 😭 — drivers still need ensure to protect concurrent calls to functions iterating those lists, namely drm_gpuvm_prepare_objects and drm_gpuvm_validate.

🚩 the DRM_GPUVM_RESV_PROTECTED option-ish thing

alternatively, drivers can set the DRM_GPUVM_RESV_PROTECTED flag to indicate that the corresponding dma_resv locks are held in order to protect the lists. if DRM_GPUVM_RESV_PROTECTED is set, internal locking is disabled and the corresponding lockdep checks are enabled.

bestie this is an optimization for drivers which are capable of taking the corresponding dma_resv locks and hence do not require internal locking. so like, only if you can actually do the thing, okay? 🙈

✨ SEE ALSO ✨

here's the whole friend group, fr fr — kernel file ./drivers/gpu/drm/drm_gpuvm.c:

🌸 that's the whole page, bestie — shipped from ./drivers/gpu/drm, dated October 2026, and it's Locking(9) if you need to cite it in the group chat 💜 🦋

📦 where this page comes from