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Date: Sat, 25 Oct 2014 20:19:48 -0400 From: Brian Silverman <bsilver16384@...il.com> To: Thomas Gleixner <tglx@...utronix.de> Cc: Austin Schuh <austin.linux@...il.com>, LKML <linux-kernel@...r.kernel.org>, Darren Hart <darren@...art.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH] futex: fix a race condition between REQUEUE_PI and task death On Sat, 25 Oct 2014, Thomas Gleixner wrote: > > pi_state_free and exit_pi_state_list both clean up futex_pi_state's. > > exit_pi_state_list takes the hb lock first, and most callers of > > pi_state_free do too. requeue_pi didn't, which causes lots of problems. > > "causes lots of problems" is not really a good explanation of the root > cause. That wants a proper description of the race, i.e. > > CPU 0 CPU 1 > ... .... > > I'm surely someone who is familiar with that code, but it took me > quite some time to understand whats going on. The casual reader will > just go into brain spiral mode and give up. Thinking about it again, I'm no longer so sure that exit_pi_state_list is the only place that it can race against. However, I did catch that one with a particularly lucky crashdump, so I know it _can_ happen there. Is just giving an example for that good? > > static void free_pi_state(struct futex_pi_state *pi_state) > > > @@ -1558,6 +1552,14 @@ retry_private: > > ret = get_futex_value_locked(&curval, uaddr1); > > > > if (unlikely(ret)) { > > + if (flags & FLAGS_SHARED && pi_state != NULL) { > > Why is this dependend on "flags & FLAGS_SHARED"? The shared/private > property has nothing to do with that at all, but I might be missing > something. Nothing... Good catch. It was a bad rebase. Thanks, Brian -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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