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From:   Mike Christie <michael.christie@oracle.com>
To:     oleg@redhat.com, linux@leemhuis.info, nicolas.dichtel@6wind.com,
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Cc:     Mike Christie <michael.christie@oracle.com>
Subject: [RFC PATCH 1/8] signal: Dequeue SIGKILL even if SIGNAL_GROUP_EXIT/group_exec_task is set
Date:   Wed, 17 May 2023 19:09:13 -0500
Message-Id: <20230518000920.191583-2-michael.christie@oracle.com>
In-Reply-To: <20230518000920.191583-1-michael.christie@oracle.com>
References: <20230518000920.191583-1-michael.christie@oracle.com>
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This has us deqeue SIGKILL even if SIGNAL_GROUP_EXIT/group_exec_task is
set when we are dealing with PF_USER_WORKER tasks.

When a vhost_task gets a SIGKILL, we could have outstanding IO in flight.
We can easily stop new work/IO from being queued to the vhost_task, but
for IO that's already been sent to something like the block layer we
need to wait for the response then process it. These type of IO
completions use the vhost_task to process the completion so we can't
exit immediately.

We need to handle wait for then handle those completions from the
vhost_task, but when we have a SIGKLL pending, functions like
schedule() return immediately so we can't wait like normal. Functions
like vhost_worker() degrade to just a while(1); loop.

This patch has get_signal drop down to the normal code path when
SIGNAL_GROUP_EXIT/group_exec_task is set so the caller can still detect
there is a SIGKILL but still perform some blocking cleanup.

Note that in that chunk I'm now bypassing that does:

sigdelset(&current->pending.signal, SIGKILL);

we look to be ok, because in the places we set SIGNAL_GROUP_EXIT/
group_exec_task we are already doing that on the threads in the
group.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
---
 kernel/signal.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 8f6330f0e9ca..ae4972eea5db 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2705,9 +2705,18 @@ bool get_signal(struct ksignal *ksig)
 		struct k_sigaction *ka;
 		enum pid_type type;
 
-		/* Has this task already been marked for death? */
-		if ((signal->flags & SIGNAL_GROUP_EXIT) ||
-		     signal->group_exec_task) {
+		/*
+		 * Has this task already been marked for death?
+		 *
+		 * If this is a PF_USER_WORKER then the task may need to do
+		 * extra work that requires waiting on running work, so we want
+		 * to dequeue the signal below and tell the caller its time to
+		 * start its exit procedure. When the work has completed then
+		 * the task will exit.
+		 */
+		if (!(current->flags & PF_USER_WORKER) &&
+		    ((signal->flags & SIGNAL_GROUP_EXIT) ||
+		     signal->group_exec_task)) {
 			clear_siginfo(&ksig->info);
 			ksig->info.si_signo = signr = SIGKILL;
 			sigdelset(&current->pending.signal, SIGKILL);
@@ -2861,11 +2870,11 @@ bool get_signal(struct ksignal *ksig)
 		}
 
 		/*
-		 * PF_IO_WORKER threads will catch and exit on fatal signals
+		 * PF_USER_WORKER threads will catch and exit on fatal signals
 		 * themselves. They have cleanup that must be performed, so
 		 * we cannot call do_exit() on their behalf.
 		 */
-		if (current->flags & PF_IO_WORKER)
+		if (current->flags & PF_USER_WORKER)
 			goto out;
 
 		/*
-- 
2.25.1