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b=NMthvpE85U9ktoGypO5aXgVOvsiNnQJg6BI9xr8TjMrL4ZuOIQTDaAf4iEJeC8VaK 0HHSl+CT3tltOR8TSbcX0OhUQodvndaalZDPLP7PckFy1dHsIU3jUG/wMgBdcWA+bn oBJiRV2jCeRmFCL7xMqo/Rjq0cJGtS7b8H+ydOWSUfEN6/Rr/WwUmjSvwqoVigVjUE OUS8o8eZS50uvpGHQZIng7zUIk7o2ZDVHShxrZr/04q47nPVcVCA+ooLJ89c9PXqvl Nm9xZZ6bcjcfxKdGuAgu9DxcJc8t+sB6VGnt7kR0qtjzpgEOERS9KyA08EG0YERP5/ amwTIbJxOsY7A== From: SeongJae Park To: Cc: SeongJae Park , "# 6 . 14 . x" , Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [RFC PATCH v3 1/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock Date: Fri, 27 Mar 2026 07:26:03 -0700 Message-ID: <20260327142605.4834-2-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260327142605.4834-1-sj@kernel.org> References: <20260327142605.4834-1-sj@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Stat-Signature: wo8rphzgr5gcnwfyxrmj1z7pc9mc83gk X-Rspamd-Queue-Id: 78807140013 X-Rspam-User: X-HE-Tag: 1774621570-211040 X-HE-Meta: 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 Yi94i32c GKB42 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: When kdamond_fn() main loop is finished, the function cancels all remaining damon_call() requests and unset the damon_ctx->kdamond so that API callers can show the context is terminated. damon_call() adds the caller's request to the queue first. After that, it shows if the kdamond of the damon_ctx is still running (damon_ctx->kdamond is set). Only if the kdamond is running, damon_call() starts waiting for the kdamond's handling of the newly added request. The damon_call() requests registration and damon_ctx->kdamond unset are protected by different mutexes, though. Hence, damon_call() could race with damon_ctx->kdamond unset, and result in deadlocks. For example, let's suppose kdamond successfully finished the damon_call() requests cancelling. Right after that, damon_call() is called for the context. It registers the new request, and shows the context is still running, because damon_ctx->kdamond unset is not yet done. Hence the damon_call() caller starts waiting for the handling of the request. However, the kdamond is already on the termination steps, so it never handles the new request. As a result, the damon_call() caller threads infinitely waits. Fix this by introducing another damon_ctx field, namely call_controls_obsolete. It is protected by the damon_ctx->call_controls_lock, which protects damon_call() requests registration. Initialize (unset) it in kdamond_fn() before letting damon_start() returns and set it just before the cancelling of remaining damon_call() requests is executed. damon_call() reads the obsolete field under the lock and avoids adding a new request. After this change, only requests that are guaranteed to be handled or cancelled are registered. Hence the after-registration DAMON context termination check is no longer needed. Remove it together. Note that the deadlock will not happen when damon_call() is called for repeat mode request. In tis case, damon_call() returns instead of waiting for the handling when the request registration succeeds and it shows the kdamond is running. However, if the request also has dealloc_on_cancel, the request memory would be leaked. The issue is found by sashiko [1]. [1] https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org Fixes: 42b7491af14c ("mm/damon/core: introduce damon_call()") Cc: # 6.14.x Signed-off-by: SeongJae Park --- include/linux/damon.h | 1 + mm/damon/core.c | 45 ++++++++++++++----------------------------- 2 files changed, 15 insertions(+), 31 deletions(-) diff --git a/include/linux/damon.h b/include/linux/damon.h index d9a3babbafc1..5129de70e7b7 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -818,6 +818,7 @@ struct damon_ctx { /* lists of &struct damon_call_control */ struct list_head call_controls; + bool call_controls_obsolete; struct mutex call_controls_lock; struct damos_walk_control *walk_control; diff --git a/mm/damon/core.c b/mm/damon/core.c index db6c67e52d2b..9bcda2765ac9 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -1573,35 +1573,6 @@ int damon_kdamond_pid(struct damon_ctx *ctx) return pid; } -/* - * damon_call_handle_inactive_ctx() - handle DAMON call request that added to - * an inactive context. - * @ctx: The inactive DAMON context. - * @control: Control variable of the call request. - * - * This function is called in a case that @control is added to @ctx but @ctx is - * not running (inactive). See if @ctx handled @control or not, and cleanup - * @control if it was not handled. - * - * Returns 0 if @control was handled by @ctx, negative error code otherwise. - */ -static int damon_call_handle_inactive_ctx( - struct damon_ctx *ctx, struct damon_call_control *control) -{ - struct damon_call_control *c; - - mutex_lock(&ctx->call_controls_lock); - list_for_each_entry(c, &ctx->call_controls, list) { - if (c == control) { - list_del(&control->list); - mutex_unlock(&ctx->call_controls_lock); - return -EINVAL; - } - } - mutex_unlock(&ctx->call_controls_lock); - return 0; -} - /** * damon_call() - Invoke a given function on DAMON worker thread (kdamond). * @ctx: DAMON context to call the function for. @@ -1619,6 +1590,10 @@ static int damon_call_handle_inactive_ctx( * synchronization. The return value of the function will be saved in * &damon_call_control->return_code. * + * Note that this function should be called only after damon_start() with the + * @ctx has succeeded. Otherwise, this function could fall into an indefinite + * wait. + * * Return: 0 on success, negative error code otherwise. */ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control) @@ -1629,10 +1604,12 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control) INIT_LIST_HEAD(&control->list); mutex_lock(&ctx->call_controls_lock); + if (ctx->call_controls_obsolete) { + mutex_unlock(&ctx->call_controls_lock); + return -ECANCELED; + } list_add_tail(&control->list, &ctx->call_controls); mutex_unlock(&ctx->call_controls_lock); - if (!damon_is_running(ctx)) - return damon_call_handle_inactive_ctx(ctx, control); if (control->repeat) return 0; wait_for_completion(&control->completion); @@ -2952,6 +2929,9 @@ static int kdamond_fn(void *data) pr_debug("kdamond (%d) starts\n", current->pid); + mutex_lock(&ctx->call_controls_lock); + ctx->call_controls_obsolete = false; + mutex_unlock(&ctx->call_controls_lock); complete(&ctx->kdamond_started); kdamond_init_ctx(ctx); @@ -3062,6 +3042,9 @@ static int kdamond_fn(void *data) damon_destroy_targets(ctx); kfree(ctx->regions_score_histogram); + mutex_lock(&ctx->call_controls_lock); + ctx->call_controls_obsolete = true; + mutex_unlock(&ctx->call_controls_lock); kdamond_call(ctx, true); damos_walk_cancel(ctx); -- 2.47.3