[RFC, 2.6.26.2-rc1] posix timers: release_posix_timer: kill the bogus put_task_struct(->it_process)

[Oliver Pinter <pinter.oliver.villany@gmail.com>]

It is an RFC for sending this patch for stable, when this patch needed, then send ACK and CC stable,
if not then send NAK.

---
>From 96347e7759e2e433c427defa0fa1adfc8cce6226 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@tv-sign.ru>
Date: Fri, 25 Jul 2008 01:47:27 -0700
Subject: [PATCH] posix timers: release_posix_timer: kill the bogus put_task_struct(->it_process);

[ Upstream commit 96347e7759e2e433c427defa0fa1adfc8cce6226 ]

release_posix_timer() can't be called with ->it_process != NULL.  Once
sys_timer_create() sets ->it_process it must not call
release_posix_timer(), otherwise we can race with another thread doing
sys_timer_delete(), this timer is visible to idr_find() and unlocked.

The same is true for two other callers (actually, for any possible
caller), sys_timer_delete() and itimer_delete().  They must clear
->it_process before unlock_timer() + release_posix_timer().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CC: Oliver Pinter <oliver.pntr@gmail.com>

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 17f5326..9a21681 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -449,9 +449,6 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
 		spin_unlock_irqrestore(&idr_lock, flags);
 	}
 	sigqueue_free(tmr->sigq);
-	if (unlikely(tmr->it_process) &&
-	    tmr->it_sigev_notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
-		put_task_struct(tmr->it_process);
 	kmem_cache_free(posix_timers_cache, tmr);
 }