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Re: [Qemu-devel] [PATCH RFC] main loop: fix some accesses made in sighan

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH RFC] main loop: fix some accesses made in sighandler context
Date: Thu, 15 Sep 2011 14:16:51 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110516 Lightning/1.0b2 Thunderbird/3.1.10

On 09/15/2011 12:22 PM, Laszlo Ersek wrote:
Make variables volatile ("sig_atomic_t" should cover "int" and "pid_t").

Also replace calls to functions that are not required to be async-signal-safe
[1]. (I haven't checked if any signal masks and/or previous suspension of the
interrupted thread keep the current calls safe.)

   ->  qemu_system_killed(): shutdown_signal, shutdown_pid, no_shutdown [2]
     ->  qemu_system_shutdown_request(): shutdown_requested
       ->  qemu_notify_event()
         ->  qemu_event_increment(): fprintf(), strerror(), exit()

[2] http://lists.nongnu.org/archive/html/qemu-devel/2011-09/msg01757.html

"checkpatch.pl" warned four times about "volatile", and considered the
zero-initialization of "no_shutdown" (which has static storage duration) an

Build tested only. Please CC me on any followup, I'm not subscribed. Thank you.

Signed-off-by: Laszlo Ersek<address@hidden>
  cpus.c   |   13 ++++++++++---
  sysemu.h |    2 +-
  vl.c     |    6 +++---
  3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/cpus.c b/cpus.c
index 54c188c..ed51247 100644
--- a/cpus.c
+++ b/cpus.c
@@ -289,9 +289,16 @@ static void qemu_event_increment(void)

      /* EAGAIN is fine, a read must be pending.  */
      if (ret<  0&&  errno != EAGAIN) {
-        fprintf(stderr, "qemu_event_increment: write() failed: %s\n",
-                strerror(errno));
-        exit (1);
+        int len;
+        char buf[128];
+        /* Don't bother with strerror_[rl]. Make a single attempt to write. */
+        len = snprintf(buf, sizeof buf,
+                       "qemu_event_increment: write() failed: %d\n", errno);

I don't think you can rely on snprintf being signal safe. I think you should just exit on failure.

OpenBSD lists snprintf as signal safe, but "probably not on other systems."


Anthony Liguori

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