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Date: Sun, 17 Sep 2023 22:19:12 +0200
From: Mirsad Todorovac <mirsad.todorovac@....unizg.hr>
To: linux-fsdevel@...r.kernel.org
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
linux-kernel@...r.kernel.org
Subject: BUG: KCSAN: data-race in poll_schedule_timeout.constprop.0 / pollwake
Hi,
The setup is Linux 6.6.0-rc1-kcsan-00269-ge789286468a9-dirty x86_64 ("dirty" is from the applied patch,
but we'll come to that below) on an Ubuntu 22.04 LTS system.
OK, so the KCSAN stacktrace is:
[ 139.315774] ==================================================================
[ 139.315798] BUG: KCSAN: data-race in poll_schedule_timeout.constprop.0 / pollwake
[ 139.315830] write to 0xffffc90003f3fb60 of 4 bytes by task 1848 on cpu 6:
[ 139.315843] pollwake+0xc0/0x110
[ 139.315860] __wake_up_common+0x7a/0x150
[ 139.315877] __wake_up_common_lock+0x7f/0xd0
[ 139.315893] __wake_up_sync_key+0x20/0x50
[ 139.315905] sock_def_readable+0x67/0x160
[ 139.315917] unix_stream_sendmsg+0x35f/0x990
[ 139.315932] sock_sendmsg+0x15d/0x170
[ 139.315947] ____sys_sendmsg+0x3d5/0x500
[ 139.315962] ___sys_sendmsg+0x9e/0x100
[ 139.315976] __sys_sendmsg+0x6f/0x100
[ 139.315990] __x64_sys_sendmsg+0x47/0x60
[ 139.316005] do_syscall_64+0x5d/0xa0
[ 139.316022] entry_SYSCALL_64_after_hwframe+0x6e/0xd8
[ 139.316043] read to 0xffffc90003f3fb60 of 4 bytes by task 1877 on cpu 18:
[ 139.316055] poll_schedule_timeout.constprop.0+0x4e/0xc0
[ 139.316071] do_sys_poll+0x50d/0x760
[ 139.316081] __x64_sys_poll+0x5f/0x210
[ 139.316091] do_syscall_64+0x5d/0xa0
[ 139.316105] entry_SYSCALL_64_after_hwframe+0x6e/0xd8
[ 139.316125] value changed: 0x00000000 -> 0x00000001
[ 139.316143] Reported by Kernel Concurrency Sanitizer on:
[ 139.316153] CPU: 18 PID: 1877 Comm: gdbus Tainted: G L 6.6.0-rc1-kcsan-00269-ge789286468a9-dirty #3
[ 139.316167] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
[ 139.316177] ==================================================================
The problematic code seems to be:
185 static int __pollwake(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
186 {
187 struct poll_wqueues *pwq = wait->private;
188 DECLARE_WAITQUEUE(dummy_wait, pwq->polling_task);
189
190 /*
191 * Although this function is called under waitqueue lock, LOCK
192 * doesn't imply write barrier and the users expect write
193 * barrier semantics on wakeup functions. The following
194 * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
195 * and is paired with smp_store_mb() in poll_schedule_timeout.
196 */
197 smp_wmb();
198 pwq->triggered = 1;
199
200 /*
201 * Perform the default wake up operation using a dummy
202 * waitqueue.
203 *
204 * TODO: This is hacky but there currently is no interface to
205 * pass in @sync. @sync is scheduled to be removed and once
206 * that happens, wake_up_process() can be used directly.
207 */
→ 208 return default_wake_function(&dummy_wait, mode, sync, key);
209 }
210
211 static int pollwake(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
212 {
213 struct poll_table_entry *entry;
214
215 entry = container_of(wait, struct poll_table_entry, wait);
216 if (key && !(key_to_poll(key) & entry->key))
217 return 0;
→ 218 return __pollwake(wait, mode, sync, key);
219 }
220
.
.
.
236
237 static int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
238 ktime_t *expires, unsigned long slack)
239 {
240 int rc = -EINTR;
241
242 set_current_state(state);
→ 243 if (!pwq->triggered)
244 rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
245 __set_current_state(TASK_RUNNING);
246
247 /*
248 * Prepare for the next iteration.
249 *
250 * The following smp_store_mb() serves two purposes. First, it's
251 * the counterpart rmb of the wmb in pollwake() such that data
252 * written before wake up is always visible after wake up.
253 * Second, the full barrier guarantees that triggered clearing
254 * doesn't pass event check of the next iteration. Note that
255 * this problem doesn't exist for the first iteration as
256 * add_wait_queue() has full barrier semantics.
257 */
258 smp_store_mb(pwq->triggered, 0);
259
260 return rc;
261 }
This quick and dirty fix had removed the KCSAN warnings which were about +100 in the test run:
You will probably have the clearer big picture, as this fix only removes the symptom.
I should probably do some homework before submitting a formal patch.
Best regards,
Mirsad Todorovac
-----------------------------------------------------------------------------------
fs/select.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/select.c b/fs/select.c
index 0ee55af1a55c..38e12084daf1 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -240,7 +240,7 @@ static int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
int rc = -EINTR;
set_current_state(state);
- if (!pwq->triggered)
+ if (!READ_ONCE(pwq->triggered))
rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
__set_current_state(TASK_RUNNING);
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