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b=uTukcn8dQ36D/HHne38+LBRSK/7+gdDAwVZFKcv/OKNn8LqRyqd06O+ooPx6t1RNY FpB20+Q2KTHs9H/Hmfnriw4hQfTuellJe6yJJY32tmhLorS5GW7hBTf4i37nHoEjyL 6CQQp9fsA8N5L4je/6umM8HvN2CfkrPi53a0iGK7dQm0qqImDBNxkJ9cwnKh13DHQI PtD2mJBH78Cm926mlTEqJVyRRsqRw9AhVdXVBZm0srt2al6zotqnOatAlSL7StgDLt iVAdvOLqY+MkGPcjd+VeqBiWyz8YBS9gVi31wOiituBRQEgdzIXQBCepMY2hWxBi8/ x6BAZuqYffsAQ== From: SeongJae Park To: SeongJae Park Cc: "# 6 . 14 . x" , Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: (sashiko review) [RFC PATCH v2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock Date: Thu, 26 Mar 2026 19:20:13 -0700 Message-ID: <20260327022014.66567-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260327021546.66441-1-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: CB40DA000F X-Stat-Signature: z7b1ddi7a6woqwsk9hjpp9huk4sqfpoy X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1774578016-350735 X-HE-Meta: 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 zWPwSgrs 7CRaZeQQ4UvqwkKEGdwsCOiW3/S4li89ZrtOXHlGvhSHmdVxFTHd6HFGu2o/XfaMny/ol7FrAXBhI09VXW+uw3ywdz5tEtOZcu4IGeeCAue2Z28vB9/D2m6ot7My+zxywYLorocbORq7RQtQIY2IO4hTJvG7gUf3YMMMEVzAXjd0I2lRpnbBp0E2uttOdMLHg2LnGtn9H15h4gmhY6j9K1yj+sPTbDfIpyTJ114HUjKPWraQQdWuiLoYjtM/d7J3tV98LXNrYw591usF7BKaOsoK3jHFoGokGsQ5Aur+s9Qx32+E= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 26 Mar 2026 19:15:45 -0700 SeongJae Park wrote: > Forwarding Sashiko review for doing discussions via mails. > > # review url: https://sashiko.dev/#/patchset/20260327004952.58266-1-sj@kernel.org > # start of sashiko.dev inline review > commit 36b78ee39968830aca3d375219a2cfce73d0d1d0 > Author: SeongJae Park > > mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock > > This commit fixes a race condition between damon_call and the > kdamond_fn exit path that could result in a deadlock. It introduces a > call_controls_obsolete flag to prevent damon_call from waiting > indefinitely for a terminating thread. > > Link: https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org > > > diff --git a/mm/damon/core.c b/mm/damon/core.c > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > > [ ... ] > > > @@ -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); > > Does damos_walk suffer from the same exit race regression that this patch > fixes for damon_call? > > In kdamond_fn, the thread termination sequence executes damos_walk_cancel > and only later sets ctx->kdamond = NULL under the kdamond_lock. > > damos_walk appears to use the same pattern as the old damon_call where it > registers ctx->walk_control, checks damon_is_running (which verifies > ctx->kdamond != NULL), and then unconditionally blocks on wait_for_completion. > > If a user triggers a DAMOS walk in the exact window after damos_walk_cancel > has finished but before ctx->kdamond = NULL is executed, wouldn't damos_walk > successfully register its control and pass the damon_is_running check? > > If so, it would then block indefinitely on wait_for_completion because the > cancellation phase has already passed and the thread is exiting. > > Because damos_walk is invoked via state_store with the global > damon_sysfs_lock held, could this infinite wait block the entire DAMON sysfs > interface for all users on the system? Yes, and I'm working on it separately, as I also replied [1] to the previous version. [1] https://lore.kernel.org/20260326135146.90670-1-sj@kernel.org Thanks, SJ [...]