linux-kernel - Re: [PATCH] Revert "random: Mix cputime from each thread that exits to the pool"

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Date:	Mon, 4 Mar 2013 12:05:08 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Russ Dill <russ.dill@...il.com>
Cc:	stable@...nel.org, linux-kernel <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Kossifidis <mickflemm@...il.com>
Subject: Re: [PATCH] Revert "random: Mix cputime from each thread that exits
 to the pool"

Russ, how about this patch instead?  Can you verify whether or not
this fixes your issue?  If so, I'll push it to Linus.

     	   		       	    - Ted

>From 24c598b5abd466f2bab954d0cc8c225e0d3a2a74 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@....edu>
Date: Mon, 4 Mar 2013 11:59:12 -0500
Subject: [PATCH] random: fix locking dependency with the tasklist_lock

Commit 6133705494bb introduced a circular lock dependency because
posix_cpu_timers_exit() is called by release_task(), which is holding
a writer lock on tasklist_lock, and this can cause a deadlock since
kill_fasync() gets called with nonblocking_pool.lock taken.

There's no reason why kill_fasync() needs to be taken while the random
pool is locked, so move it out to fix this locking dependency.

Signed-off-by: "Theodore Ts'o" <tytso@....edu>
Reported-by: Russ Dill <Russ.Dill@...il.com>
Cc: stable@...nel.org
---
 drivers/char/random.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 85e81ec..57d4b15 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -852,6 +852,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		      int reserved)
 {
 	unsigned long flags;
+	int wakeup_write = 0;
 
 	/* Hold lock while accounting */
 	spin_lock_irqsave(&r->lock, flags);
@@ -873,10 +874,8 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		else
 			r->entropy_count = reserved;
 
-		if (r->entropy_count < random_write_wakeup_thresh) {
-			wake_up_interruptible(&random_write_wait);
-			kill_fasync(&fasync, SIGIO, POLL_OUT);
-		}
+		if (r->entropy_count < random_write_wakeup_thresh)
+			wakeup_write = 1;
 	}
 
 	DEBUG_ENT("debiting %zu entropy credits from %s%s\n",
@@ -884,6 +883,11 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 
 	spin_unlock_irqrestore(&r->lock, flags);
 
+	if (wakeup_write) {
+		wake_up_interruptible(&random_write_wait);
+		kill_fasync(&fasync, SIGIO, POLL_OUT);
+	}
+
 	return nbytes;
 }
 
-- 
1.7.12.rc0.22.gcdd159b

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