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b=Mwy694kr/IucXYv3o4lC2wcGmle08x19rR8AKqu+ApQaw1MttHlYRk1xDjfyvAMw9 g2ZTIW+1aX1dNWinkhZL2TqZFnXF4AjEM72Emb9B6XENOWmz/RoEIMBR0cYzgZ8NYB P51XxGpZlLA24yflwmaciqrNigaY0I9oZqLfTyaTvAE9pnA3iAYxKCw41j1y90ex0A Mzc+Dun0MZ+33dOmRdR3UGFoAI/+8ZhqhMDP3YDdLcbPkWT909w+OALReQcZFsZl6g CU2V7a3Myj83O5kpFlhcUzCAHR4QKJFszZRj3nJ8DPGYb5nyTm2039NzMHA0B5GLVC tEJRvg6BSBrwQ== 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 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock Date: Wed, 25 Mar 2026 23:23:46 -0700 Message-ID: <20260326062347.88569-3-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260326062347.88569-1-sj@kernel.org> References: <20260326062347.88569-1-sj@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 4qysu5m6zbgfpg7c1s6in9f9yyxr44yo X-Rspamd-Queue-Id: 0ED0420004 X-Rspamd-Server: rspam09 X-HE-Tag: 1774506235-614990 X-HE-Meta: U2FsdGVkX1/gjKVR4O1/Nndt1CE28ZxZVmxxashEIFEn699RtcLgqA8mLnLL3VnAO5zoh0ESiEmXTWB30vSBWlwalba7nc1X/8vxqZUrFZUco2/dTJ8+Yl/KqI3tN/zdLubVbfyqWmxWvwnJAebqy5ntdPEQjTbo0LcshQA3gf6UcMFluu8uSX6+fYAoKkLXqa0vrdYSb/8yFumDGA6lzOgOEFYaL1qM6GOcJtug5fpxKIImjaoVNJxB4OgRni8fHViFh/gQb/xeRemjYTGKQ3A16NeARDxukq3di5Ic1i4jzA8kQOzW5W5SPAxFU/ep3EMVQa4WoY07fHZckwcx4viHL9EbjGXrRK2TuprmyIwAlBgp2MNHFvGFm37awx39jrcrTpRBjoOvcZ1Dyhu/KEqk69altAZfLQ776YlLzBhn8sk6TIcZYyo4X+IFPdK5Ck7SSIFEvy+ZzBvQj5ivPjKWBNCARTUoH+/hs3HTV6enx61zvmevHcjRuIvDDBjyfLC5DO69ci6hKvKo++tjhaXTLhToAmkdxdAlmzv8uOhiRh+T3WehyGsxj3HO6tb+hvJbP5zctae7ugv7J2cxtUt0urIU+NjpSQS0/Xiig+XOyBnq/veEwKLVGLOFdvyn0qog1qwy39B2fZdjzCqiPXgE5WlQwTMAO6bgAjbmuiCObzJeGvs7rGxO5lWEW0tWWS/meCivaMA2f9PaIM3sYSU3vL23M94bYc3PL7Rr2UxiDqyDuOQOk/XfHi44GQs+0WjVdNQN2suwEXnDdJVXsm/ParEu1B5EZijFywv3LxcwGJcrzS0Px7inm+btjYr8dmSPbqcDJRhy4aEGMFY/dM9RNKgV8/N79cWEh31Mqfa8jaf4nezyXutXXQkci44YKULBnQ9jLxgTtji4AMG868Vy5wmd6hi9Yrmc71o2yjFuUfGl3QbxQIVq0Z12F9VEWfz9op1NKgUh8lj6CDP phMdq+9u aR6DTZ6zYOYMmxBWJ++zj5cELnmyPtcj8+Hk8E/mZNXbe0DnYqzGOrnNyI40seoIqsPxBkDBUcaMkZi8FERVM4F5bjbcQvltN39gDIXpHm4oWhwrmX+bZLpje3rgQSAAJjEs9oMOY+BkU6gz2V1V+qsMfeUY6oY161dB3KRFQpB9n5cjPvFXj1FdEq97zvJWviQCtlhOifEvMJZTib2GBdQdzDGg0OoJNz/ugn5Uoyb0QwTCM4hkO1EGyQPnMZiTso8y5jqM7bvS8ir0i0+zjwN5+AA== 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 is 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 results 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 show 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 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() registration. Initialize (unset) it in kdamond_init_ctx() and set just before the cancelling of remaining damon_call() is executed. damon_call() reads the obsolete field under the lock and avoid adding a new request. After this change, only requests that guaranteed to be handled or cancelled are registered. Hence the after-registration DAMON context termination check is no more needed. Remove it together. 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 | 41 ++++++++++------------------------------- 2 files changed, 11 insertions(+), 31 deletions(-) diff --git a/include/linux/damon.h b/include/linux/damon.h index d9a3babbafc16..5129de70e7b70 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 db6c67e52d2b8..a2b553e2c5a81 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. @@ -1629,10 +1600,12 @@ 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); @@ -2939,6 +2912,9 @@ static void kdamond_init_ctx(struct damon_ctx *ctx) damos_set_next_apply_sis(scheme, ctx); damos_set_filters_default_reject(scheme); } + mutex_lock(&ctx->call_controls_lock); + ctx->call_controls_obsolete = false; + mutex_unlock(&ctx->call_controls_lock); } /* @@ -3062,6 +3038,9 @@ 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