linux-kernel - Re: [PATCH V5] futex: set FLAGS_HAS_TIMEOUT during futex

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Message-ID: <4DA85114.3010501@linux.intel.com>
Date:	Fri, 15 Apr 2011 07:07:16 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, John Kacur <jkacur@...hat.com>,
	stable@...nel.org
Subject: Re: [PATCH V5] futex: set FLAGS_HAS_TIMEOUT during futex_wait restart
 setup



On 04/15/2011 01:33 AM, Thomas Gleixner wrote:
> On Thu, 14 Apr 2011, Darren Hart wrote:
> 
>> The FLAGS_HAS_TIMEOUT flag was not getting set, causing the restart_block to
>> restart futex_wait() without a timeout after a signal.
>>
>> Commit b41277dc7a18ee332d in 2.6.38 introduced the regression by accidentally
>> removing the the FLAGS_HAS_TIMEOUT assignment from futex_wait() during the setup
>> of the restart block. Restore the originaly behavior.
>>
>> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=32922
>>
>> V2: Added references to commit message.
>> V3: Set flag during restart block instead of do_futex()
>> V4: Correct stupid order of assignment mistake pointed out by Eric
>> V5: Correct subject to match implementation, correct stable submission
>>
>> Signed-off-by: Darren Hart <dvhart@...ux.intel.com>
>> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
>> Reported-by: Tim Smith <tsmith201104@...oo.com>
>> Reported-by: Torsten Hilbrich <torsten.hilbrich@...unet.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...e.hu>
>> Cc: John Kacur <jkacur@...hat.com>
>> Cc: stable@...nel.org
>> ---
>>  kernel/futex.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/futex.c b/kernel/futex.c
>> index bda4157..abd5324 100644
>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -1886,7 +1886,7 @@ retry:
>>  	restart->futex.val = val;
>>  	restart->futex.time = abs_time->tv64;
>>  	restart->futex.bitset = bitset;
>> -	restart->futex.flags = flags;
>> +	restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
> 
> We only get here when a timeout is pending. So why don't we just do
> the obvious:
> 
> --- linux-2.6.orig/kernel/futex.c
> +++ linux-2.6/kernel/futex.c
> @@ -1902,16 +1902,13 @@ out:
>  static long futex_wait_restart(struct restart_block *restart)
>  {
>  	u32 __user *uaddr = restart->futex.uaddr;
> -	ktime_t t, *tp = NULL;
> +	ktime_t t;
>  
> -	if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
> -		t.tv64 = restart->futex.time;
> -		tp = &t;
> -	}
> +	t.tv64 = restart->futex.time;
>  	restart->fn = do_no_restart_syscall;
>  
>  	return (long)futex_wait(uaddr, restart->futex.flags,
> -				restart->futex.val, tp, restart->futex.bitset);
> +				restart->futex.val, &t, restart->futex.bitset);
>  }

I believe I asked you the same question when I was adding the
FLAGS_HAS_TIMEOUT. :-) The problem is distinguishing between no timeout
and an expired timer. The above always passes a non-null address for
abs_time to futex_wait(), which will then always schedule a timer.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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