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b=MpnKQCIthhpYQViR4eiqkUx/JtLQkvYNH98ArsRlc2fFcawW/XWjcUs1J9HRlmv9T zuHJ6cgbL6jGdPob9bmcLVbHKHqfd/H07BJ9NLqZ26xQAxomx40F2N8CVx/Vm8sJCG Roq4PNNdlU6JSoRdWOB4+WsQ5cVrghM78yCKnseBH6TMvnhhjCuAMg/ZPyDSNwsfbX E85fF1Z500VMuaUWGvoBJCyBay2Fr5EicQwwqjHw/QFfH0pgRxMirVz/uhpiCDRskX ICbYVgCnqKmGly8UdFU5kD/iwFHOfy4XL+mtgrT0RO63lNgCl9E69Fa38QadwzpX3v nb35zI3oOR5hg== 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 20:53:54 -0700 Message-ID: <20260327035355.68068-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260327022014.66567-1-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 1suu13cj7gribrrkhdc1jg4g6wsy7dj3 X-Rspamd-Queue-Id: C1B96140008 X-Rspamd-Server: rspam09 X-HE-Tag: 1774583637-218282 X-HE-Meta: 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 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 19:20:13 -0700 SeongJae Park wrote: > 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. Maybe I will make the next version of this patch as a series of two patches, one for this one and the other one for the damos_walk() race fix. > > [1] https://lore.kernel.org/20260326135146.90670-1-sj@kernel.org Thanks, SJ [...]