RTS/IOManager: fix trac issue #9722.

Summary:
Whenever the RTS has been inactive for idleGCDelayTime, the idle timer
fires and calls wakeUpRts(), which in turn calls ioManagerWakeup(),
which in turn writes a byte (or a few) to a file descriptor (stored in
the io_manager_wakeup_fd variable) registered by the TimerManager and
on which the TimerManager will wait. (Note that the write will only
occur if the file descriptor is non-negative.) When the RTS shuts
down, it shuts down the TimerManager, and in this process the file
descriptor stored in io_manager_wakeup_fd is closed. In the error
case, the idle timer fires after the close of the file occurs, and
then the write() call in ioManagerWakeup() fails and the
aforementioned error message gets printed.

This patch solves the problem by (1) having the TimerManager (via
Control) write -1 to io_manager_wakeup_fd just before closing the file
descriptor written in io_manager_wakeup_fd, and (2) having
ioManagerWakeup() ignore an error returned by write() in the case that
the write returned -1 and the io_manager_wakeup_fd is -1.

Reviewers: austin, simonmar, hvr, thomie

Reviewed By: thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D722

GHC Trac Issues: #9722

1 files changed, 15 insertions, 2 deletions

diff --git a/rts/posix/Signals.c b/rts/posix/Signals.c
index 5fbb917cf8..a2fa07f091 100644
--- a/rts/posix/Signals.c
+++ b/rts/posix/Signals.c
@@ -126,7 +126,7 @@ more_handlers(int sig)
 }
 
 // Here's the pipe into which we will send our signals
-static int io_manager_wakeup_fd = -1;
+static volatile int io_manager_wakeup_fd = -1;
 static int timer_manager_control_wr_fd = -1;
 
 #define IO_MANAGER_WAKEUP 0xff
@@ -161,7 +161,20 @@ ioManagerWakeup (void)
         StgWord8 byte = (StgWord8)IO_MANAGER_WAKEUP;
         r = write(io_manager_wakeup_fd, &byte, 1);
 #endif
-        if (r == -1) { sysErrorBelch("ioManagerWakeup: write"); }
+        /* N.B. If the TimerManager is shutting down as we run this
+         * then there is a possiblity that our first read of
+         * io_manager_wakeup_fd is non-negative, but before we get to the
+         * write the file is closed. If this occurs, io_manager_wakeup_fd
+         * will be written into with -1 (GHC.Event.Control does this prior
+         * to closing), so checking this allows us to distinguish this case.
+         * To ensure we observe the correct ordering, we declare the
+         * io_manager_wakeup_fd as volatile.
+         * Since this is not an error condition, we do not print the error
+         * message in this case.
+         */
+        if (r == -1 && io_manager_wakeup_fd >= 0) {
+            sysErrorBelch("ioManagerWakeup: write");
+        }
     }
 }