linux-kernel - Re: [PATCH] fix for zeroed user-space tids in multi-threaded core dumps

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Date:	Thu, 20 Aug 2015 16:48:35 +0200
From:	Andreas Schwab <schwab@...e.de>
To:	Roland McGrath <roland@...k.frob.com>
Cc:	Ernie Petrides <petrides@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	Jakub Jelinek <jakub@...hat.com>,
	"Ulrich Drepper \<drepper.fsp\@gmail.com\>\, Ingo Molnar" 
	<mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix for zeroed user-space tids in multi-threaded core dumps

Roland McGrath <roland@...hat.com> writes:

> [PATCH] Disable CLONE_CHILD_CLEARTID for abnormal exit.
>
> The CLONE_CHILD_CLEARTID flag is used by NPTL to have its threads
> communicate via memory/futex when they exit, so pthread_join can
> synchronize using a simple futex wait.  The word of user memory where NPTL
> stores a thread's own TID is what it passes; this gets reset to zero at
> thread exit.
>
> It is not desireable to touch this user memory when threads are dying due
> to a fatal signal.  A core dump is more usefully representative of the
> dying program state if the threads live at the time of the crash have their
> NPTL data structures unperturbed.  The userland expectation of
> CLONE_CHILD_CLEARTID has only ever been that it works for a thread making
> an _exit system call.

This breaks nscd.  It uses CLONE_CHILD_CLEARTID to clear the
nscd_certainly_running flag in the shared databases, so that the clients
are notified when nscd is restarted.  Now, when nscd uses a
non-persistent database, the clients that have it mapped keep thinking
the database is being updated by nscd, when in fact nscd has created a
new (anonymous) one (for non-persistent databases it uses an unlinked
file as backend).

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@...e.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
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