Re: [Intel-gfx] [PATCH] drm/i915: Mark the GEM context link as RCU protected
On 07/01/2020 11:15, Tvrtko Ursulin wrote: On 22/12/2019 23:35, Chris Wilson wrote: The only protection for intel_context.gem_cotext is granted by RCU, so annotate it as a rcu protected pointer and carefully dereference it in the few occasions we need to use it. Fixes: 9f3ccd40acf4 ("drm/i915: Drop GEM context as a direct link from i915_request") Signed-off-by: Chris Wilson Cc: Andi Shyti --- drivers/gpu/drm/i915/gem/i915_gem_context.c | 5 ++- drivers/gpu/drm/i915/gt/intel_context_types.h | 2 +- drivers/gpu/drm/i915/gt/intel_reset.c | 26 +--- .../gpu/drm/i915/gt/intel_ring_submission.c | 2 +- drivers/gpu/drm/i915/i915_gpu_error.c | 40 --- drivers/gpu/drm/i915/i915_request.c | 6 +-- drivers/gpu/drm/i915/i915_request.h | 8 7 files changed, 62 insertions(+), 27 deletions(-) [snip] static void engine_record_requests(struct intel_engine_cs *engine, @@ -1298,28 +1304,34 @@ static void error_record_engine_execlists(const struct intel_engine_cs *engine, static bool record_context(struct drm_i915_error_context *e, const struct i915_request *rq) { - const struct i915_gem_context *ctx = rq->context->gem_context; + struct i915_gem_context *ctx; + struct task_struct *task; + bool capture; + rcu_read_lock(); + ctx = rcu_dereference(rq->context->gem_context); + if (ctx && !kref_get_unless_zero(&ctx->ref)) + ctx = NULL; + rcu_read_unlock(); if (!ctx) return false; - if (ctx->pid) { - struct task_struct *task; - - rcu_read_lock(); - task = pid_task(ctx->pid, PIDTYPE_PID); - if (task) { - strcpy(e->comm, task->comm); - e->pid = task->pid; - } - rcu_read_unlock(); + rcu_read_lock(); + task = pid_task(ctx->pid, PIDTYPE_PID); + if (task) { + strcpy(e->comm, task->comm); + e->pid = task->pid; } + rcu_read_unlock(); Why is this rcu_read_lock section needed? The first one obtained the reference to the context so should be good. Hmm pid_task() does: ... first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]), lockdep_tasklist_lock_is_held()); ... Note, tasklist_lock_is_held. Regards, Tvrtko ___ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
Re: [Intel-gfx] [PATCH] drm/i915: Mark the GEM context link as RCU protected
Quoting Tvrtko Ursulin (2020-01-07 11:15:39) > > On 22/12/2019 23:35, Chris Wilson wrote: > > The only protection for intel_context.gem_cotext is granted by RCU, so > > annotate it as a rcu protected pointer and carefully dereference it in > > the few occasions we need to use it. > > > > Fixes: 9f3ccd40acf4 ("drm/i915: Drop GEM context as a direct link from > > i915_request") > > Signed-off-by: Chris Wilson > > Cc: Andi Shyti > > --- > > drivers/gpu/drm/i915/gem/i915_gem_context.c | 5 ++- > > drivers/gpu/drm/i915/gt/intel_context_types.h | 2 +- > > drivers/gpu/drm/i915/gt/intel_reset.c | 26 +--- > > .../gpu/drm/i915/gt/intel_ring_submission.c | 2 +- > > drivers/gpu/drm/i915/i915_gpu_error.c | 40 --- > > drivers/gpu/drm/i915/i915_request.c | 6 +-- > > drivers/gpu/drm/i915/i915_request.h | 8 > > 7 files changed, 62 insertions(+), 27 deletions(-) > > > > [snip] > > > > > static void engine_record_requests(struct intel_engine_cs *engine, > > @@ -1298,28 +1304,34 @@ static void error_record_engine_execlists(const > > struct intel_engine_cs *engine, > > static bool record_context(struct drm_i915_error_context *e, > > const struct i915_request *rq) > > { > > - const struct i915_gem_context *ctx = rq->context->gem_context; > > + struct i915_gem_context *ctx; > > + struct task_struct *task; > > + bool capture; > > > > + rcu_read_lock(); > > + ctx = rcu_dereference(rq->context->gem_context); > > + if (ctx && !kref_get_unless_zero(&ctx->ref)) > > + ctx = NULL; > > + rcu_read_unlock(); > > if (!ctx) > > return false; > > > > - if (ctx->pid) { > > - struct task_struct *task; > > - > > - rcu_read_lock(); > > - task = pid_task(ctx->pid, PIDTYPE_PID); > > - if (task) { > > - strcpy(e->comm, task->comm); > > - e->pid = task->pid; > > - } > > - rcu_read_unlock(); > > + rcu_read_lock(); > > + task = pid_task(ctx->pid, PIDTYPE_PID); > > + if (task) { > > + strcpy(e->comm, task->comm); > > + e->pid = task->pid; > > } > > + rcu_read_unlock(); > > Why is this rcu_read_lock section needed? The first one obtained the > reference to the context so should be good. The task returned by ctx->pid is not protected by that reference, and pid_task() doesn't increment the reference on the task. That's what I remember of the pid_task() interface, that requires rcu to be held while you look inside, where get_pid_task() does not. -Chris ___ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
Re: [Intel-gfx] [PATCH] drm/i915: Mark the GEM context link as RCU protected
On 22/12/2019 23:35, Chris Wilson wrote: The only protection for intel_context.gem_cotext is granted by RCU, so annotate it as a rcu protected pointer and carefully dereference it in the few occasions we need to use it. Fixes: 9f3ccd40acf4 ("drm/i915: Drop GEM context as a direct link from i915_request") Signed-off-by: Chris Wilson Cc: Andi Shyti --- drivers/gpu/drm/i915/gem/i915_gem_context.c | 5 ++- drivers/gpu/drm/i915/gt/intel_context_types.h | 2 +- drivers/gpu/drm/i915/gt/intel_reset.c | 26 +--- .../gpu/drm/i915/gt/intel_ring_submission.c | 2 +- drivers/gpu/drm/i915/i915_gpu_error.c | 40 --- drivers/gpu/drm/i915/i915_request.c | 6 +-- drivers/gpu/drm/i915/i915_request.h | 8 7 files changed, 62 insertions(+), 27 deletions(-) [snip] static void engine_record_requests(struct intel_engine_cs *engine, @@ -1298,28 +1304,34 @@ static void error_record_engine_execlists(const struct intel_engine_cs *engine, static bool record_context(struct drm_i915_error_context *e, const struct i915_request *rq) { - const struct i915_gem_context *ctx = rq->context->gem_context; + struct i915_gem_context *ctx; + struct task_struct *task; + bool capture; + rcu_read_lock(); + ctx = rcu_dereference(rq->context->gem_context); + if (ctx && !kref_get_unless_zero(&ctx->ref)) + ctx = NULL; + rcu_read_unlock(); if (!ctx) return false; - if (ctx->pid) { - struct task_struct *task; - - rcu_read_lock(); - task = pid_task(ctx->pid, PIDTYPE_PID); - if (task) { - strcpy(e->comm, task->comm); - e->pid = task->pid; - } - rcu_read_unlock(); + rcu_read_lock(); + task = pid_task(ctx->pid, PIDTYPE_PID); + if (task) { + strcpy(e->comm, task->comm); + e->pid = task->pid; } + rcu_read_unlock(); Why is this rcu_read_lock section needed? The first one obtained the reference to the context so should be good. Regards, Tvrtko ___ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx