Messages in this thread Patch in this message | | | Date | Sun, 17 Sep 2023 22:19:12 +0200 | From | Mirsad Todorovac <> | Subject | BUG: KCSAN: data-race in poll_schedule_timeout.constprop.0 / pollwake |
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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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