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If we don't process SIGCHLD before another comes, we will see just one
SIGCHLD as a result. In this case current code will miss exit
notification for a session and wait forever.

Adding extra waitpid check for all sessions when SIGCHLD is received, to
make sure we don't miss any session exit.

Also fix close condition for signal_fd.
Reported-by: Ian Rogers <irogers@google.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210320221013.1619613-1-jolsa@kernel.org

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>