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Thu, 27 Aug 2026 08:02:32 -0700 (PDT) From: Sunho Park To: paulmck@kernel.org Cc: jiangshanlai@gmail.com, josh@joshtriplett.org, work@onurozkan.dev, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, qiang.zhang@linux.dev, frederic@kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Sunho Park , syzbot+d4faf7db59e11f6fd1ab@syzkaller.appspotmail.com Subject: [PATCH] srcu: Fix WARN_ON() for rcu_segcblist_n_cbs() in cleanup_srcu_struct() Date: Fri, 28 Aug 2026 00:02:08 +0900 Message-ID: <20260827150208.2166775-1-shpark061104@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The WARN_ON() added by commit 78a38cbf6f20 ("srcu: Queue sdp->work when the delay timer is successfully deleted") uses rcu_segcblist_n_cbs() to detect callbacks that srcu_barrier() failed to wait for. However, the ->len counter is decremented only at the end of srcu_invoke_callbacks(), after the invoking loop has finished. Since srcu_barrier() can return right after the barrier callback is invoked, cleanup_srcu_struct() can see a non-zero n_cbs even though the cblist is already physically empty, falsely triggering the WARN_ON() together with a still-pending delay_work timer. Use rcu_segcblist_empty(), which checks the actual head of the cblist. Callbacks that have genuinely not been invoked yet still leave the list non-empty, so the WARN_ON() still catches callers that skip srcu_barrier() or queue callbacks after it. Link: https://lore.kernel.org/rcu/e6350377085ddd85d6ef00d8e9a67bd50c762d3c@linux.dev/T/#t Reported-by: syzbot+d4faf7db59e11f6fd1ab@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=d4faf7db59e11f6fd1ab Fixes: 78a38cbf6f20 ("srcu: Queue sdp->work when the delay timer is successfully deleted") Signed-off-by: Sunho Park --- kernel/rcu/srcutree.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index ed204b3f4b84..ad27880dd690 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -704,7 +704,7 @@ void cleanup_srcu_struct(struct srcu_struct *ssp) // Call srcu_barrier() before this cleanup_srcu_struct() // to avoid triggering this WARN_ON(). if (WARN_ON(timer_delete_sync(&sdp->delay_work) && - rcu_segcblist_n_cbs(&sdp->srcu_cblist)) && + !rcu_segcblist_empty(&sdp->srcu_cblist)) && rcu_cpu_beenfullyonline(sdp->cpu)) queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work); flush_work(&sdp->work); -- 2.43.0