The patch titled
Subject: kernel/fork: fix CLONE_CHILD_CLEARTID regression in nscd
has been removed from the -mm tree. Its filename was
kernel-fork-fix-clone_child_cleartid-regression-in-nscd.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Michal Hocko <mhocko@xxxxxxxx>
Subject: kernel/fork: fix CLONE_CHILD_CLEARTID regression in nscd
fec1d0115240 ("[PATCH] Disable CLONE_CHILD_CLEARTID for abnormal exit")
has caused a subtle regression in nscd which 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, 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).
The original proposal for the CLONE_CHILD_CLEARTID change claimed
(https://lkml.org/lkml/2006/10/25/233):
: The NPTL library uses the CLONE_CHILD_CLEARTID flag on clone() syscalls
: on behalf of pthread_create() library calls. This feature is used to
: request that the kernel clear the thread-id in user space (at an address
: provided in the syscall) when the thread disassociates itself from the
: address space, which is done in mm_release().
:
: Unfortunately, when a multi-threaded process incurs a core dump (such as
: from a SIGSEGV), the core-dumping thread sends SIGKILL signals to all of
: the other threads, which then proceed to clear their user-space tids
: before synchronizing in exit_mm() with the start of core dumping. This
: misrepresents the state of process's address space at the time of the
: SIGSEGV and makes it more difficult for someone to debug NPTL and glibc
: problems (misleading him/her to conclude that the threads had gone away
: before the fault).
:
: The fix below is to simply avoid the CLONE_CHILD_CLEARTID action if a
: core dump has been initiated.
The resulting patch from Roland (https://lkml.org/lkml/2006/10/26/269)
seems to have a larger scope than the original patch asked for. It seems
that limitting the scope of the check to core dumping should work for
SIGSEGV issue describe above.
[Changelog partly based on Andreas' description]
Fixes: fec1d0115240 ("[PATCH] Disable CLONE_CHILD_CLEARTID for abnormal exit")
Link: http://lkml.kernel.org/r/1471968749-26173-1-git-send-email-mhocko@xxxxxxxxxx
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Tested-by: William Preston <wpreston@xxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Roland McGrath <roland@xxxxxxxxxxxxx>
Cc: Andreas Schwab <schwab@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/fork.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)
diff -puN kernel/fork.c~kernel-fork-fix-clone_child_cleartid-regression-in-nscd kernel/fork.c
--- a/kernel/fork.c~kernel-fork-fix-clone_child_cleartid-regression-in-nscd
+++ a/kernel/fork.c
@@ -913,14 +913,12 @@ void mm_release(struct task_struct *tsk,
deactivate_mm(tsk, mm);
/*
- * If we're exiting normally, clear a user-space tid field if
- * requested. We leave this alone when dying by signal, to leave
- * the value intact in a core dump, and to save the unnecessary
- * trouble, say, a killed vfork parent shouldn't touch this mm.
- * Userland only wants this done for a sys_exit.
+ * Signal userspace if we're not exiting with a core dump
+ * because we want to leave the value intact for debugging
+ * purposes.
*/
if (tsk->clear_child_tid) {
- if (!(tsk->flags & PF_SIGNALED) &&
+ if (!(tsk->signal->flags & SIGNAL_GROUP_COREDUMP) &&
atomic_read(&mm->mm_users) > 1) {
/*
* We don't check the error code - if userspace has
_
Patches currently in -mm which might be from mhocko@xxxxxxxx are
mm-vmscan-get-rid-of-throttle_vm_writeout.patch
oom-keep-mm-of-the-killed-task-available.patch
kernel-oom-fix-potential-pgd_lock-deadlock-from-__mmdrop.patch
mm-oom-get-rid-of-signal_struct-oom_victims.patch
oom-suspend-fix-oom_killer_disable-vs-pm-suspend-properly.patch
mm-make-sure-that-kthreads-will-not-refault-oom-reaped-memory.patch
oom-oom_reaper-allow-to-reap-mm-shared-by-the-kthreads.patch
oom-warn-if-we-go-oom-for-higher-order-and-compaction-is-disabled.patch
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