kcsan: Fix misreporting if concurrent races on same address

If there are at least 4 threads racing on the same address, it can
happen that one of the readers may observe another matching reader in
other_info. To avoid locking up, we have to consume 'other_info'
regardless, but skip the report. See the added comment for more details.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/kcsan/report.c

@@ -422,6 +422,44 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
 			return false;
 		}
 
+		access_type |= other_info.access_type;
+		if ((access_type & KCSAN_ACCESS_WRITE) == 0) {
+			/*
+			 * While the address matches, this is not the other_info
+			 * from the thread that consumed our watchpoint, since
+			 * neither this nor the access in other_info is a write.
+			 * It is invalid to continue with the report, since we
+			 * only have information about reads.
+			 *
+			 * This can happen due to concurrent races on the same
+			 * address, with at least 4 threads. To avoid locking up
+			 * other_info and all other threads, we have to consume
+			 * it regardless.
+			 *
+			 * A concrete case to illustrate why we might lock up if
+			 * we do not consume other_info:
+			 *
+			 *   We have 4 threads, all accessing the same address
+			 *   (or matching address ranges). Assume the following
+			 *   watcher and watchpoint consumer pairs:
+			 *   write1-read1, read2-write2. The first to populate
+			 *   other_info is write2, however, write1 consumes it,
+			 *   resulting in a report of write1-write2. This report
+			 *   is valid, however, now read1 populates other_info;
+			 *   read2-read1 is an invalid conflict, yet, no other
+			 *   conflicting access is left. Therefore, we must
+			 *   consume read1's other_info.
+			 *
+			 * Since this case is assumed to be rare, it is
+			 * reasonable to omit this report: one of the other
+			 * reports includes information about the same shared
+			 * data, and at this point the likelihood that we
+			 * re-report the same race again is high.
+			 */
+			release_report(flags, KCSAN_REPORT_RACE_SIGNAL);
+			return false;
+		}
+
 		/*
 		 * Matching & usable access in other_info: keep other_info_lock
 		 * locked, as this thread consumes it to print the full report;