sim, kern: support FUTEX_CMP_REQUEUE

This patch supports FUTEX_CMP_REQUEUE operation. Below is its
description from Linux man page:

futex syscall: int futex(int *uaddr, int futex_op, int val,
                         const struct timespec *timeout,
                         int *uaddr2, int val3);

This operation first checks whether the location uaddr still contains
the value val3.  If not, the operation fails with the error EAGAIN.
Otherwise, the operation wakes up a maximum of val waiters that are
waiting on the futex at uaddr.  If there are more than val waiters, then
the remaining waiters are removed from the wait queue of the source
futex at uaddr and added to the wait queue of the target futex at
uaddr2.  The val2 argument specifies an upper limit on the number of
waiters that are requeued to the futex at uaddr2.

Reference: http://man7.org/linux/man-pages/man2/futex.2.html

Change-Id: I6d2ebd19a935b656d19d8342f7ab450c0d2031f4
Reviewed-on: https://gem5-review.googlesource.com/c/9629
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/src/sim/syscall_emul.hh b/src/sim/syscall_emul.hh
index 295598c52..0b7585cb4 100644
--- a/src/sim/syscall_emul.hh
+++ b/src/sim/syscall_emul.hh
@@ -441,8 +441,21 @@ futexFunc(SyscallDesc *desc, int callnum, Process *process,
         return futex_map.wakeup(uaddr, process->tgid(), val);
     } else if (OS::TGT_FUTEX_WAKE_BITSET == op) {
         return futex_map.wakeup_bitset(uaddr, process->tgid(), val3);
-    }
+    } else if (OS::TGT_FUTEX_REQUEUE == op ||
+               OS::TGT_FUTEX_CMP_REQUEUE == op) {
 
+        // Ensure futex system call accessed atomically.
+        BufferArg buf(uaddr, sizeof(int));
+        buf.copyIn(tc->getMemProxy());
+        int mem_val = *(int*)buf.bufferPtr();
+        /*
+         * For CMP_REQUEUE, the whole operation is only started only if
+         * val3 is still the value of the futex pointed to by uaddr.
+         */
+        if (OS::TGT_FUTEX_CMP_REQUEUE && val3 != mem_val)
+            return -OS::TGT_EWOULDBLOCK;
+        return futex_map.requeue(uaddr, process->tgid(), val, timeout, uaddr2);
+    }
     warn("futex: op %d not implemented; ignoring.", op);
     return -ENOSYS;
 }