linux-kernel - Re: [PATCH 2/3] livepatch: send a fake signal to all blocking tasks

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Message-ID: <20170518164909.GA939@redhat.com>
Date:   Thu, 18 May 2017 18:49:10 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jeyu@...hat.com, jikos@...nel.org,
        pmladek@...e.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] livepatch: send a fake signal to all blocking tasks

I didn't see other patches in series, not sure I understand...

On 05/18, Miroslav Benes wrote:
>
> The very safe marking is done in entry.S on syscall and
> interrupt/exception exit paths, and in a stack checking functions of
> livepatch.  TIF_PATCH_PENDING is cleared and the next
> recalc_sigpending() drops TIF_SIGPENDING.

Confused. The task can't return from do_signal() is signal_pending() is
true, thus it will spin forever if klp_patch_pending(current)) is true.
"forever" means until something else clears TIF_PATCH_PENDING, of course.

exit_to_usermode_loop() calls do_signal(), then klp_update_patch_state().
So it won't be cleared here.

Even if you change the order, this won't help unless I missed something,
TIF_PATCH_PENDING can be set when this task has already entered do_signal().

> Last, sending the fake signal is not automatic. It is done only when
> admin requests it by writing 1 to force sysfs attribute in livepatch
> sysfs directory.

OK, but see above, even if klp_send_fake_signal() is never called, the
a task will get this fake signal when it calls recalc_sigpending().

Oleg.