On Thu, Jan 28, 2016 at 03:18:23PM +1100, Sam Bobroff wrote: > On Linux, if QEMU is run from a shell with -d to enable debug logging > but without directing it to a file (e.g. -D is not used, and > qemu_logfile is set to stderr), and no shell redirection is used, it > is possible for log messages to be lost under load. > > This is caused by a combination of several factors: > > * The shell (e.g. bash) may provide stdin, stdout and stderr as > duplicates of a single open file, so they share file status flags > including O_NONBLOCK. > > * As character devices are registered (see qemu_chr_open_stdio()), > stdin and stdout are set non-blocking. > > * The printf() family of functions, including fprintf(), are not > "non-blocking" aware: if they receive EAGAIN after partially > writing their output, they immediately return EAGAIN with no way to > discover how much output was written, if any. > > So O_NONBLOCK is set on stdin/stdout, which causes O_NONBLOCK to be > set on stderr, and qemu_logfile is set to stderr. Then, under load, > fprintf()s to qemu_logfile return EAGAIN, which is ignored and data is > lost. This can't be fixed by handling EAGAIN because an unknown amount > of data has been written. > > This patch works around the issue by re-opening the underlying tty > file, which is available in Linux as /proc/self/fd/2, and duplicating > it into fd 2 which causes stderr to refer to a new, unshared, "file > description". Additionally, if we end up logging to a non-blocking > file descriptor on a non-Linux platform, where no workaround has been > implemented, we display a warning message. > > Signed-off-by: Sam Bobroff <sam.bobr...@au1.ibm.com> > --- > This is somewhat of an RFC as the issue probably hasn't been noticed > very often (ever?) and there are several approaches to addressing it. > The discussion was a while ago, here: > > http://lists.nongnu.org/archive/html/qemu-devel/2015-12/msg00761.html > > include/sysemu/os-posix.h | 2 +- > include/sysemu/os-win32.h | 2 +- > os-posix.c | 30 +++++++++++++++++++++++++++++- > os-win32.c | 2 +- > vl.c | 2 +- > 5 files changed, 33 insertions(+), 5 deletions(-)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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