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Received: from authenticated-user by stravinsky.debian.org with esmtpsa (TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.96) (envelope-from ) id 1wXyPV-00Adru-3A; Fri, 12 Jun 2026 09:42:46 +0000 Date: Fri, 12 Jun 2026 02:42:41 -0700 From: Breno Leitao To: SeongJae Park Cc: Catalin Marinas , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH RFC] mm/kmemleak: avoid soft lockup when scanning task stacks Message-ID: References: <20260611-kmemleak-stack-resched-v1-1-d6248ade5f4a@debian.org> <20260612011049.84146-1-sj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260612011049.84146-1-sj@kernel.org> X-Debian-User: leitao X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 71DD1100003 X-Stat-Signature: jce8557oqruunuxtk7umj5pnk89wbocd X-HE-Tag: 1781257371-659026 X-HE-Meta: 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 LP+pf4Ax sffkWQrY945JZtOy+hbBcFQduOiOUvF0JJZr+hGOzSfw89BZ0RFG3rwyNa3VJB6sxpR7rbq87npma8cudpsLkxdYlYQex+99ooWAuLJEqUq8PaCASJpo6UU6tRRIb0GsW6NSW9fwPnS7Eax3rRCbzrpvn7dzyZLKIw119Bf3IBBAqw4ezM0h8h5SMUMMY+ML77Vp0ajsvTadse80SOjOMKrY1nueZooouqupAY1Lpnu7HStWfN8WdnwYcx51H8alqDwOT9+PkWs/bVtQoXTnfUtpMBO2u1yeVLH+JYEboxoEB4D2Ct/NBpILK3w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 11, 2026 at 06:10:48PM -0700, SeongJae Park wrote: > On Thu, 11 Jun 2026 05:45:00 -0700 Breno Leitao wrote: > > > kmemleak_scan() walks every thread and scans its kernel stack under a > > single rcu_read_lock() with no reschedule point. On a host with very > > many threads -- amplified by KASAN/lockdep in debug builds -- this loop > > can hog a CPU long enough to trip the soft lockup watchdog: > > > > watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] > > scan_block > > kmemleak_scan > > kmemleak_scan_thread > > kthread > > > > A cond_resched() cannot be added directly: the loop runs inside an RCU > > read-side critical section. > > > > Split the scan in two parts: > > > > 1) get the list of tasks (with RCU read lock) in an array > > 2) run scan_block() for the tasks (with cond_reschd()). > > > > Is it a sane approach? > > > > Signed-off-by: Breno Leitao > > --- > > mm/kmemleak.c | 26 ++++++++++++++++++++++---- > > 1 file changed, 22 insertions(+), 4 deletions(-) > > > > diff --git a/mm/kmemleak.c b/mm/kmemleak.c > > index 7c7ba17ce7af0..9f8a35ecbb50c 100644 > > --- a/mm/kmemleak.c > > +++ b/mm/kmemleak.c > > @@ -62,6 +62,7 @@ > > #include > > #include > > #include > > +#include > > #include > > #include > > #include > > @@ -1885,17 +1886,34 @@ static void kmemleak_scan(void) > > * Scanning the task stacks (may introduce false negatives). > > */ > > if (kmemleak_stack_scan) { > > - struct task_struct *p, *g; > > + struct task_struct **tasks, *p, *g; > > + unsigned int nr = 0, max, i; > > > > + max = nr_threads + 64; > > + tasks = kvmalloc_array(max, sizeof(*tasks), GFP_KERNEL); > > + > > + /* Snapshot the threads under RCU */ > > rcu_read_lock(); > > for_each_process_thread(g, p) { > > - void *stack = try_get_task_stack(p); > > + if (!tasks || nr >= max) > > + break; > > Why don't you check !tasks right after the allocation? Good question. I will update if we agree this approach is good enough. Thanks for the review, SJ! --breno